Support POSIX threading model for MINGW

[fire]

The use of godot crosscompiled on Linux requires POSIX threading model.

They suggest:

To use posix mode for mingw by default:

$ sudo update-alternatives --config x86_64-w64-mingw32-gcc
<choose x86_64-w64-mingw32-gcc-posix from the list>
$ sudo update-alternatives --config x86_64-w64-mingw32-g++
<choose x86_64-w64-mingw32-g++-posix from the list>

(godotengine/godot#9258)

I'm not sure how to do this for wclang.

[fire]

$ x86_64-w64-mingw32-gcc-posix -v
Using built-in specs.
COLLECT_GCC=x86_64-w64-mingw32-gcc-posix
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-w64-mingw32/5.3-posix/lto-wrapper
Target: x86_64-w64-mingw32
Configured with: ../../src/configure --build=x86_64-linux-gnu --prefix=/usr --includedir='/usr/include' --mandir='/usr/s
hare/man' --infodir='/usr/share/info' --sysconfdir=/etc --localstatedir=/var --disable-silent-rules --libexecdir='/usr/l
ib/gcc-mingw-w64' --disable-maintainer-mode --disable-dependency-tracking --prefix=/usr --enable-shared --enable-static
--disable-multilib --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --libdir=/usr/lib --enable-libstd
cxx-time=yes --with-tune=generic --enable-version-specific-runtime-libs --enable-fully-dynamic-string --enable-libgomp -
-enable-languages=c,c++,fortran,objc,obj-c++ --enable-lto --with-plugin-ld --enable-threads=posix --program-suffix=-posi
x --program-prefix=x86_64-w64-mingw32- --target=x86_64-w64-mingw32 --with-as=/usr/bin/x86_64-w64-mingw32-as --with-ld=/u
sr/bin/x86_64-w64-mingw32-ld --disable-libatomic
Thread model: posix
gcc version 5.3.1 20160211 (GCC)

$ x86_64-w64-mingw32-gcc-win32 -v
Using built-in specs.
COLLECT_GCC=x86_64-w64-mingw32-gcc-win32
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-w64-mingw32/5.3-win32/lto-wrapper
Target: x86_64-w64-mingw32
Configured with: ../../src/configure --build=x86_64-linux-gnu --prefix=/usr --includedir='/usr/include' --mandir='/usr/s
hare/man' --infodir='/usr/share/info' --sysconfdir=/etc --localstatedir=/var --disable-silent-rules --libexecdir='/usr/l
ib/gcc-mingw-w64' --disable-maintainer-mode --disable-dependency-tracking --prefix=/usr --enable-shared --enable-static
--disable-multilib --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --libdir=/usr/lib --enable-libstd
cxx-time=yes --with-tune=generic --enable-version-specific-runtime-libs --enable-fully-dynamic-string --enable-libgomp -
-enable-languages=c,c++,fortran,objc,obj-c++ --enable-lto --with-plugin-ld --enable-threads=win32 --program-suffix=-win3
2 --program-prefix=x86_64-w64-mingw32- --target=x86_64-w64-mingw32 --with-as=/usr/bin/x86_64-w64-mingw32-as --with-ld=/u
sr/bin/x86_64-w64-mingw32-ld --disable-libatomic
Thread model: win32
gcc version 5.3.1 20160211 (GCC)

[fire]

clang version 4.0.0-1ubuntu1~16.04.2 (tags/RELEASE_400/rc1)
Target: x86_64-w64-windows-gnu
Thread model: posix
InstalledDir: /usr/bin
 "/usr/lib/llvm-4.0/bin/clang" -cc1 -triple x86_64-w64-windows-gnu -emit-obj -mrelax-all -disable-free -disable-llvm-verifier -discard-value-names -main-file-name hello.cpp -mrelocation-model pic -pic-level 2 -mthread-model posix -fmath-errno -masm-verbose -mconstructor-aliases -munwind-tables -target-cpu x86-64 -momit-leaf-frame-pointer -v -dwarf-column-info -debugger-tuning=gdb -nostdsysteminc -nobuiltininc -resource-dir /usr/lib/llvm-4.0/bin/../lib/clang/4.0.0 -isystem /usr/bin/../lib/clang/4.0/include -isystem /usr/x86_64-w64-mingw32/include/../../../usr/lib/gcc/x86_64-w64-mingw32/5.3-win32/include/c++ -isystem /usr/x86_64-w64-mingw32/include/../../../usr/lib/gcc/x86_64-w64-mingw32/5.3-win32/include/c++/x86_64-w64-mingw32 -isystem /usr/x86_64-w64-mingw32/include -fdeprecated-macro -fdebug-compilation-dir /home/fire -ferror-limit 19 -fmessage-length 120 -fno-use-cxa-atexit -fobjc-runtime=gcc -fcxx-exceptions -fexceptions -fdiagnostics-show-option -fcolor-diagnostics -o /tmp/hello-c55cd5.o -x c++ hello.cpp
clang -cc1 version 4.0.0 based upon LLVM 4.0.0 default target x86_64-pc-linux-gnu
#include "..." search starts here:
#include <...> search starts here:
 /usr/bin/../lib/clang/4.0/include
 /usr/x86_64-w64-mingw32/include/../../../usr/lib/gcc/x86_64-w64-mingw32/5.3-win32/include/c++
 /usr/x86_64-w64-mingw32/include/../../../usr/lib/gcc/x86_64-w64-mingw32/5.3-win32/include/c++/x86_64-w64-mingw32
 /usr/x86_64-w64-mingw32/include
End of search list.
 "/usr/bin/x86_64-w64-mingw32-ld" -m i386pep -Bdynamic -o hello /usr/x86_64-w64-mingw32/lib/crt2.o /usr/x86_64-w64-mingw32/lib/crtbegin.o -L/usr/lib/gcc/x86_64-w64-mingw32/5.3-posix -L/usr/x86_64-w64-mingw32/lib -L/usr/lib -L/usr/x86_64-w64-mingw32/sys-root/mingw/lib /tmp/hello-c55cd5.o -lmingw32 -lgcc -lgcc_eh -lmoldname -lmingwex -lmsvcrt -ladvapi32 -lshell32 -luser32 -lkernel32 -lmingw32 -lgcc -lgcc_eh -lmoldname -lmingwex -lmsvcrt /usr/x86_64-w64-mingw32/lib/crtend.o
/tmp/hello-c55cd5.o:(.text+0xc): undefined reference to `std::ios_base::Init::Init()'
/tmp/hello-c55cd5.o:(.text+0x3c): undefined reference to `std::ios_base::Init::~Init()'
/tmp/hello-c55cd5.o:(.text+0x62): undefined reference to `std::cout'
/tmp/hello-c55cd5.o:(.text+0x75): undefined reference to `std::basic_ostream<char, std::char_traits<char> >& std::operator<< <std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*)'
clang: error: linker command failed with exit code 1 (use -v to see invocation)```

`/usr/lib/gcc/x86_64-w64-mingw32/5.3-win32` is incorrect.

Should be `/usr/lib/gcc/x86_64-w64-mingw32/5.3-posix`.

[fire]

I gave up and just deleted /usr/lib/gcc/x86_64-w64-mingw32/5.3-win32. This is a clue that it's an ordering issue.