refs.c: SSE2 optimizations for check_refname_component

Optimize check_refname_component using SSE2 on x86_64.

git rev-parse HEAD is a good test-case for this, since it does almost
nothing except parse refs.  For one particular repo with about 60k
refs, almost all packed, the timings are:

Look up table: 29 ms
SSE2:          23 ms

This cuts about 20% off of the runtime.

The configure.ac changes include code from the GNU C Library written
by Joseph S. Myers <joseph at codesourcery dot com>.

Ondřej Bílka <neleai@seznam.cz> suggested an SSE2 approach to the
substring searches, which netted a speed boost.

Signed-off-by: David Turner <dturner@twitter.com>