Use CFLAGS if set as-is, match CXXFLAGS behavior

Since 2c93711, CXXFLAGS is used
as-is if set in the envionment rather than clobbered by whatever
CPython happened to be built with.

Do the same for CFLAGS: use it as-is if set in the environment, don't
prepend CPython's saved flags.

Fixes: pypa#299

distutils/sysconfig.py

@@ -340,6 +340,7 @@ def customize_compiler(compiler):
 
         ldshared = _add_flags(ldshared, 'LD')
         ldcxxshared = _add_flags(ldcxxshared, 'LD')
+        cflags = os.environ.get('CFLAGS', cflags)
         cflags = _add_flags(cflags, 'C')
         ldshared = _add_flags(ldshared, 'C')
         cxxflags = os.environ.get('CXXFLAGS', cxxflags)