windows-build.sh: ensure that -static-libgcc is used with MSYS2 gcc

windows-build.sh: ensure that -static-libgcc is used with MSYS2 gcc

In MSYS2 MINGW32, the gcc version is "i686-w64-mingw32-cc.exe (Rev4,
Built by MSYS2 project) 13.2.0" which does not contain the string
"(GCC)" as expected.  Similarly for MSYS2 MINGW64.  This means that
-static-libgcc does not get used as intended.  As a quick fix, just
invert the check, so gcc is now detected by checking for not clang..