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>

configure.ac

@@ -895,7 +895,9 @@ AC_LANG_PROGRAM([[
 #include <pthread.h>
 ]], [[
 	pthread_mutex_t test_mutex;
+	pthread_key_t test_key;
 	int retcode = 0;
+	retcode |= pthread_key_create(&test_key, (void *)0);
 	retcode |= pthread_mutex_init(&test_mutex,(void *)0);
 	retcode |= pthread_mutex_lock(&test_mutex);
 	retcode |= pthread_mutex_unlock(&test_mutex);