Add alias to restore 2.7.2 compatibility for setup scripts (#13994).

The customize_compiler function moved many times during the 2.7 series;
in 2.7.3, setup scripts importing this function from ccompiler were
broken.  This commit restores compatibility without reintroducing the
issue that #13994 originally fixed (duplication of the function).

A unit test makes little sense here, as distutils tests never do imports
in functions, and the fix is very simple.