Support Linux-to-Windows cross compile via MinGW

[dneto0]

NOTE: MinGW can't compile googletest, so if #190 goes in first, then we must rework this pull request.

Supports Shaderc google/shaderc#169

[dneto0]

Looking for feedback and testing from @AWoloszyn

This PR should get the WIP label, but I can't apply it.

[dneto0]

Without swapping these two, I get the following error:

FAILED: /usr/bin/i686-w64-mingw32-g++   -DGLSLANG_OSINCLUDE_WIN32 -std=c++11 -w -MMD -MT third_party/glslang/glslang/CMakeFiles/glslang.dir/MachineIndependent/PoolAlloc.cpp.obj -MF "third_party/glslang/glslang/CMakeFiles/glslang.dir/MachineIndependent/PoolAlloc.cpp.obj.d" -o third_party/glslang/glslang/CMakeFiles/glslang.dir/MachineIndependent/PoolAlloc.cpp.obj -c ../third_party/glslang/glslang/MachineIndependent/PoolAlloc.cpp
In file included from ../third_party/glslang/glslang/MachineIndependent/PoolAlloc.cpp:36:0:
../third_party/glslang/glslang/MachineIndependent/../Include/Common.h:42:22: error: previous declaration of ‘int sprintf_s(char*, size_t, const char*, ...)’ with ‘C++’ linkage
     #define snprintf sprintf_s
                      ^
In file included from /usr/share/mingw-w64/include/stdio.h:1000:0,
                 from /usr/share/mingw-w64/include/locale.h:12,
                 from /usr/include/c++/4.8/clocale:42,
                 from /usr/include/c++/4.8/i686-w64-mingw32/bits/c++locale.h:41,
                 from /usr/include/c++/4.8/bits/localefwd.h:40,
                 from /usr/include/c++/4.8/string:43,
                 from /usr/include/c++/4.8/stdexcept:39,
                 from /usr/include/c++/4.8/array:38,
                 from /usr/include/c++/4.8/tuple:39,
                 from /usr/include/c++/4.8/unordered_set:41,
                 from ../third_party/glslang/glslang/MachineIndependent/../Include/Common.h:73,
                 from ../third_party/glslang/glslang/MachineIndependent/PoolAlloc.cpp:36:
/usr/share/mingw-w64/include/sec_api/stdio_s.h:29:78: error: conflicts with new declaration with ‘C’ linkage
   int __cdecl sprintf_s(char *_DstBuf,size_t _DstSize,const char *_Format,...);
                                                                              ^
In file included from /usr/include/c++/4.8/ext/string_conversions.h:43:0,
                 from /usr/include/c++/4.8/bits/basic_string.h:2815,
                 from /usr/include/c++/4.8/string:52,
                 from /usr/include/c++/4.8/stdexcept:39,
                 from /usr/include/c++/4.8/array:38,
                 from /usr/include/c++/4.8/tuple:39,
                 from /usr/include/c++/4.8/unordered_set:41,
                 from ../third_party/glslang/glslang/MachineIndependent/../Include/Common.h:73,
                 from ../third_party/glslang/glslang/MachineIndependent/PoolAlloc.cpp:36:
/usr/include/c++/4.8/cstdio:168:11: error: ‘::snprintf’ has not been declared
   using ::snprintf;
           ^
/usr/include/c++/4.8/cstdio:178:22: error: ‘__gnu_cxx::snprintf’ has not been declared
   using ::__gnu_cxx::snprintf;
                      ^

[AWoloszyn]

I tested this compiling on Linux for Windows, and everything works correctly.

[dneto0]

Rebased against master, and @AWoloszyn retested on Windows.

No longer in DO NOT MERGE, no longer WIP.