Ok, here is a patch which fixes the issue while retaining the AR
customization. Here is what it does:

 - configure defines both AR and ARFLAGS in the configure script, and
those are used in the Makefile
 - ARFLAGS is used instead of the harcoded rc (rc is the default value
for ARFLAGS in configure)
 - Both AR and ARFLAGS are used in customize_compiler in distutils. If
any of them is customized from the environment, they are overriden.

Some examples:
 - default: nothing changes, except that archiver is set to ar rc
instead of ar in customize_compiler, thus build_clib is not broken anymore
 - setting AR/ARFLAGS in configure: those are used in the python build
 - ARFLAGS=cru python setup.py build_clib will use ar cru instead of ar
cr for libraries.

The only thing I am a bit unsure is that instead of ar -cr, we have ar
cr used as archiver in distutils. Since ac cr is currently used in the
python makefile, I guess most unixes understand ar cr, though.