use $(CC) instead of gcc

MinGW has various gcc aliases:
mingw32-gcc, i686-pc-mingw32-gcc, i686-w64-mingw32-gcc,
x86_64-w64-mingw32-gcc, etc.
This change enables to use them.
Merge pull request #411 from k-takata/topic/use-CC-instead-of-gcc

use $(CC) instead of gcc