Differences from previous patch submission:

1. Configure verifies that the compiler implements VSX builtins.
If so vector math library is built by default.

2. A runtime check prevents vector math tests running on systems
lacking VSX hardware.

3. File vec_finite_alias.c is now empty. Its existence ensures that
libmvec_nonshared.a is created. Otherwise, linking during testing
of a newly-installed glibc fails.