Added #ifdef conditions for MinGW because this compiler doesn't define
gcc atomic builtins, so __sync_val_compare_and_swap was not found
neither any of any __GCC_HAVE_SYNC_COMPARE_AND_SWAP_* definition
This fix includes some fixes for config.h.win32 that was generating
some compiling errors on MinGW64 and also fix CMakeLists.txt in order to
give better support to MinGW and MSYS2
Pablo Sanabria