C mxe linking fails: ld cannot find symbols

trying to cross-compile TranscriberAG with MXE, I'm stuck on a problem I don't understand.

I managed to reproduce the problem on a tiny file:

#include <dlfcn.h>

int test() {
   dlopen("test", RTLD_LAZY);
}

I compile it with:

/path/to/mxe/usr/bin/i686-pc-mingw32.static-g++  -o test_dl.obj -c test_dl.cpp
/path/to/mxe/usr/bin/i686-pc-mingw32.static-ar cr test_dl.a test_dl.obj
/path/to/mxe/usr/bin/i686-pc-mingw32.static-g++  -shared -o test_dl.dll -Wl,--whole-archive test_dl.a -Wl,--no-whole-archive -ldl -lkernel32 -luser32 -lgdi32 -lwinspool -lshell32 -lole32 -loleaut32 -luuid -lcomdlg32 -ladvapi32

(of course, these commands are generated by CMake, I cannot just change them simply). It fails with:

test_dl.a(test_dl.obj):test_dl.cpp:(.text+0x16): undefined reference to `_dlopen(char const*, int)'
collect2: error: ld returned 1 exit status

I have installed dlfcn in mxe. If I try a similar command with my native (Debian/Sid) compiler:

g++  -o test_dl.obj -c test_dl.cpp -fPIC
ar cr test_dl.a test_dl.obj
g++  -shared -o test_dl.dll -Wl,--whole-archive test_dl.a -Wl,--no-whole-archive -ldl

it works well...

Here is the output of objdump:

$ /home/eroux/softs/mxe/usr/i686-pc-mingw32/bin/objdump -t /home/eroux/softs/mxe/usr/i686-pc-mingw32.static/lib/libdl.a 
In archive /home/eroux/softs/mxe/usr/i686-pc-mingw32.static/lib/libdl.a:
dlfcn.o:     file format pe-i386

SYMBOL TABLE:
[  0](sec -2)(fl 0x00)(ty   0)(scl 103) (nx 1) 0x00000000 dlfcn.c
File 
[  2](sec  1)(fl 0x00)(ty  20)(scl   3) (nx 1) 0x00000000 _save_err_str
AUX tagndx 0 ttlsiz 0x0 lnnos 0 next 0
[  4](sec  3)(fl 0x00)(ty   0)(scl   3) (nx 0) 0x00000040 _error_buffer
[  5](sec  3)(fl 0x00)(ty   0)(scl   3) (nx 0) 0x00000000 _current_error
[  6](sec  1)(fl 0x00)(ty  20)(scl   2) (nx 0) 0x00000190 _dlopen
[  7](sec  3)(fl 0x00)(ty   0)(scl   3) (nx 0) 0x00010040 _first_object
[  8](sec  1)(fl 0x00)(ty  20)(scl   2) (nx 0) 0x00000300 _dlclose
[  9](sec  1)(fl 0x00)(ty  20)(scl   2) (nx 0) 0x000003a0 _dlsym
[ 10](sec  1)(fl 0x00)(ty  20)(scl   2) (nx 0) 0x00000460 _dlerror
[ 11](sec  1)(fl 0x00)(ty   0)(scl   3) (nx 1) 0x00000000 .text
AUX scnlen 0x470 nreloc 42 nlnno 0
[ 13](sec  2)(fl 0x00)(ty   0)(scl   3) (nx 1) 0x00000000 .data
AUX scnlen 0x0 nreloc 0 nlnno 0
[ 15](sec  3)(fl 0x00)(ty   0)(scl   3) (nx 1) 0x00000000 .bss
AUX scnlen 0x1004c nreloc 0 nlnno 0
[ 17](sec  4)(fl 0x00)(ty   0)(scl   3) (nx 1) 0x00000000 .rdata
AUX scnlen 0xb nreloc 0 nlnno 0
[ 19](sec  6)(fl 0x00)(ty   0)(scl   3) (nx 1) 0x00000000 .rdata$zzz
AUX scnlen 0x11 nreloc 0 nlnno 0
[ 21](sec  7)(fl 0x00)(ty   0)(scl   3) (nx 1) 0x00000000 .eh_frame
AUX scnlen 0x17c nreloc 5 nlnno 0
[ 23](sec  0)(fl 0x00)(ty  20)(scl   2) (nx 0) 0x00000000 _GetLastError@0
[ 24](sec  0)(fl 0x00)(ty  20)(scl   2) (nx 0) 0x00000000 _FormatMessageA@28
[ 25](sec  0)(fl 0x00)(ty  20)(scl   2) (nx 0) 0x00000000 _SetErrorMode@4
[ 26](sec  0)(fl 0x00)(ty  20)(scl   2) (nx 0) 0x00000000 _LoadLibraryExA@12
[ 27](sec  0)(fl 0x00)(ty  20)(scl   2) (nx 0) 0x00000000 _malloc
[ 28](sec  0)(fl 0x00)(ty  20)(scl   2) (nx 0) 0x00000000 _GetModuleHandleA@4
[ 29](sec  0)(fl 0x00)(ty  20)(scl   2) (nx 0) 0x00000000 _sprintf
[ 30](sec  0)(fl 0x00)(ty  20)(scl   2) (nx 0) 0x00000000 _FreeLibrary@4
[ 31](sec  0)(fl 0x00)(ty  20)(scl   2) (nx 0) 0x00000000 _free
[ 32](sec  0)(fl 0x00)(ty  20)(scl   2) (nx 0) 0x00000000 _GetProcAddress@8
[ 33](sec  0)(fl 0x00)(ty  20)(scl   2) (nx 0) 0x00000000 _CloseHandle@4

I must be missing something obvious... What is it?

Edit: if I add -v to the third g++ command, I get:

Using built-in specs.
COLLECT_GCC=/home/eroux/softs/mxe/usr/bin/i686-pc-mingw32.static-g++
COLLECT_LTO_WRAPPER=/home/eroux/softs/mxe/usr/libexec/gcc/i686-pc-mingw32.static/4.9.1/lto-wrapper
Target: i686-pc-mingw32.static
Configured with: /home/eroux/softs/mxe/tmp-gcc-i686-pc-mingw32.static/gcc-4.9.1/configure --target=i686-pc-mingw32.static --build=x86_64-unknown-linux-gnu --prefix=/home/eroux/softs/mxe/usr --libdir=/home/eroux/softs/mxe/usr/lib --enable-languages=c,c++,objc,fortran --enable-version-specific-runtime-libs --with-gcc --with-gnu-ld --with-gnu-as --disable-nls --disable-shared --disable-multilib --without-x [...]
Thread model: win32
gcc version 4.9.1 (GCC) 
COMPILER_PATH=/home/eroux/softs/mxe/usr/libexec/gcc/i686-pc-mingw32.static/4.9.1/:/home/eroux/softs/mxe/usr/libexec/gcc/i686-pc-mingw32.static/4.9.1/:/home/eroux/softs/mxe/usr/libexec/gcc/i686-pc-mingw32.static/:/home/eroux/softs/mxe/usr/lib/gcc/i686-pc-mingw32.static/4.9.1/:/home/eroux/softs/mxe/usr/lib/gcc/i686-pc-mingw32.static/:/home/eroux/softs/mxe/usr/lib/gcc/i686-pc-mingw32.static/4.9.1/../../../../i686-pc-mingw32.static/bin/
LIBRARY_PATH=/home/eroux/softs/mxe/usr/lib/gcc/i686-pc-mingw32.static/4.9.1/:/home/eroux/softs/mxe/usr/lib/gcc/i686-pc-mingw32.static/4.9.1/../../../../i686-pc-mingw32.static/lib/
COLLECT_GCC_OPTIONS='-v' '-shared' '-o' 'test_dl.dll' '-mtune=generic' '-march=pentiumpro'
/home/eroux/softs/mxe/usr/libexec/gcc/i686-pc-mingw32.static/4.9.1/collect2 -plugin /home/eroux/softs/mxe/usr/libexec/gcc/i686-pc-mingw32.static/4.9.1/liblto_plugin.so -plugin-opt=/home/eroux/softs/mxe/usr/libexec/gcc/i686-pc-mingw32.static/4.9.1/lto-wrapper -plugin-opt=-fresolution=/tmp/cc3LyEtG.res -plugin-opt=-pass-through=-lmingw32 -plugin-opt=-pass-through=-lgcc -plugin-opt=-pass-through=-lmoldname -plugin-opt=-pass-through=-lmingwex -plugin-opt=-pass-through=-lmsvcrt -plugin-opt=-pass-through=-ladvapi32 -plugin-opt=-pass-through=-lshell32 -plugin-opt=-pass-through=-luser32 -plugin-opt=-pass-through=-lkernel32 -plugin-opt=-pass-through=-lmingw32 -plugin-opt=-pass-through=-lgcc -plugin-opt=-pass-through=-lmoldname -plugin-opt=-pass-through=-lmingwex -plugin-opt=-pass-through=-lmsvcrt --shared -Bdynamic -e _DllMainCRTStartup@12 --enable-auto-image-base -o test_dl.dll /home/eroux/softs/mxe/usr/lib/gcc/i686-pc-mingw32.static/4.9.1/../../../../i686-pc-mingw32.static/lib/dllcrt2.o /home/eroux/softs/mxe/usr/lib/gcc/i686-pc-mingw32.static/4.9.1/crtbegin.o -L/home/eroux/softs/mxe/usr/lib/gcc/i686-pc-mingw32.static/4.9.1 -L/home/eroux/softs/mxe/usr/lib/gcc/i686-pc-mingw32.static/4.9.1/../../../../i686-pc-mingw32.static/lib --whole-archive test_dl.a --no-whole-archive -ldl -lkernel32 -luser32 -lgdi32 -lwinspool -lshell32 -lole32 -loleaut32 -luuid -lcomdlg32 -ladvapi32 -lstdc++ -lmingw32 -lgcc -lmoldname -lmingwex -lmsvcrt -ladvapi32 -lshell32 -luser32 -lkernel32 -lmingw32 -lgcc -lmoldname -lmingwex -lmsvcrt /home/eroux/softs/mxe/usr/lib/gcc/i686-pc-mingw32.static/4.9.1/crtend.o
test_dl.a(test_dl.obj):test_dl.cpp:(.text+0x16): undefined reference to `dlopen(char const*, int)'
collect2: error: ld returned 1 exit status

Which seems to indicate that the good libdl is taken, right?

Edit: Adding LD_LIBRARY_PATH='' before the commands doesn't change anything...

It's looks like the dlfcn.h header doesn't have function prototypes wrapped in a extern "C" block so it doesn't get compiled correctly with C++.

Make sure the dlfcn-win32 library that you're using is up to date (the extern "C" wrapper was added to dlfcn.h in January).