Ninja: Remove redundant check for GNU-like compiler on Windows

Update the Ninja generator's check to work using whatever language is
being enabled instead of hard-coding C and CXX.  With that, the
undocumented internal `CMAKE_COMPILER_IS_MINGW` variable is only set by
compilers already covered by other alternatives in the condition.  See
commit b3de0dfe93 (Ninja: Use forward slashes for any GCC on Windows,
2015-05-07, v3.3.0-rc1~93^2~3).