configure.ac: Generalize gcc / clang version check

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14529 a5019735-40e9-0310-863c-91ae7b9d1cf9
configure.ac: Generalize glibc version check

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14530 a5019735-40e9-0310-863c-91ae7b9d1cf9