Accept glibc 2.20 as valid.

The pre-releases were all called 2.19.90 and worked fine.
But now that glibc 2.20 final is out configure wouldn't recognize it.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14505 a5019735-40e9-0310-863c-91ae7b9d1cf9