Add autoconf tests for pthreads

Set the value of PTHREAD_LIBS to the correct flags for linking pthreads on
the current environment.

Signed-off-by: David M. Syzdek <david.syzdek@acsalaska.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/config.mak.in b/config.mak.in
index 7170729..ea7705c 100644
--- a/config.mak.in
+++ b/config.mak.in
@@ -51,3 +51,4 @@
 NO_DEFLATE_BOUND=@NO_DEFLATE_BOUND@
 FREAD_READS_DIRECTORIES=@FREAD_READS_DIRECTORIES@
 SNPRINTF_RETURNS_BOGUS=@SNPRINTF_RETURNS_BOGUS@
+PTHREAD_LIBS=@PTHREAD_LIBS@