Ratpoints for OS X 10.7 (#12368) needs review

[Jeroen Demeyer]

I added a small patch to the ratpoints spkg to make it build
with gcc on Darwin. Instead of unconditionally adding the compiler flag
-fnested-functions on Darwin, I instead only add it if supported by the
compiler. Please review:

http://trac.sagemath.org/sage_trac/ticket/12368

This is a prerequisite to building Sage on OS X 10.7 with GCC-4.6.2.

Thanks,
Jeroen.