Improve test for pthreads flag

Improve test for pthreads flag

When compiling with CC=clang using Clang 1.1 as shipped by Debian
unstable (package version 2.7-3), the -mt flag is sufficient to compile
during the `configure` test. However, building git would then fail at
link time complaining about missing symbols such as `pthread_key_create'
and `pthread_create'.

Work around this issue by adding pthread key creation to the pthreads
configure test source.

Signed-off-by: Giuseppe Bilotta <giuseppe.bilotta@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>