avoid using gcc or mingw32-gcc directly, use CC instead

When MinGW(-w64) package is installed on Cygwin, the executable name is
i686-pc-mingw32-gcc, i686-w64-mingw32-gcc or x86_64-w64-mingw32-gcc.
It's better to define CC variable and use it.

Related: #119
Merge pull request #120 from k-takata/avoid-using-gcc-directly

avoid using gcc or mingw32-gcc directly, use CC instead
Improve documentation