Trying to generate gdc for mingw-w64(x86 and x64) using msys2

[Vincent R via D.gnu]

Hi,

I would like to generate a d compiler targeting mingw-w64 using 
the msys2 build system.
So I have installed msys2 (posix emulation like cygwin) as shown 
here: http://msys2.github.io/ and I have installed necessary 
packages:

$ pacman -S base-devel
$ pacman -S mingw-w64-x86_64-toolchain

optionally you can also installed  mingw-w64-i686-toolchain if 
you want to build mingw32 packages but for now I am only 
interested in mingw64 (64 bits).

Then I have forked MINGW-packages repository 
(https://github.com/vrichomme/MINGW-packages) to make my 
experiments.

$ git clone https://github.com/vrichomme/MINGW-packages.git
$ cd MINGW-packages/mingw-w64-gcc

Inside mingw-w64-gcc I have used the 4 patches and renamed them 
to be more compliant with msys2 build system:

gdc-patch-gcc-5.patch
gdc-patch-toplev-5.patch
gdc-patch-versym-cpu-5.patch
gdc-patch-versym-os-5.patch

Then we are ready to start compilation:

$ MINGW_INSTALLS=mingw64 makepkg-mingw -sLf
...
/home/19013692/MINGW-packages/mingw-w64-gcc/src/build-x86_64-w64-mingw32/./gcc/gdc -B/home/19013692/MINGW-packages/mingw-w64-gcc/src/build-x86_64-w64-mingw32/./gcc/ -L/mingw64/x86_64-w64-mingw32/lib -L/mingw64/lib -isystem /mingw64/x86_64-w64-mingw32/include -isystem /mingw64/include -B/mingw64/x86_64-w64-mingw32/bin/ -B/mingw64/x86_64-w64-mingw32/lib/ -isystem /mingw64/x86_64-w64-mingw32/include -isystem /mingw64/x86_64-w64-mingw32/sys-include    -o gcc/gthreads/posix.o -Wall  -g -frelease -O2 -nostdinc -pipe -Wno-deprecated -I ../../../../gcc-5.2.0/libphobos/libdruntime -I ./x86_64-w64-mingw32 -I .  -c ../../../../gcc-5.2.0/libphobos/libdruntime/gcc/gthreads/posix.d
../../../../gcc-5.2.0/libphobos/libdruntime/gcc/gthreads/posix.d:29:1: error: undefined identifier pthread_key_t, did you mean alias gthread_key_t?
  alias gthread_key_t   = pthread_key_t;
  ^
...

I am quite new to D language but I can see that posix.d imports 
core.sys.posix.pthread and I thought that on Windows 
Version(Posix) is not defined so how can it compile ?
Do I need to add a -DPosix or something like that ?
Do I need to add a Version(Mingw64) inside core.sys.posix.pthread 
?

Thanks