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>

config.mak.in

@@ -51,3 +51,4 @@ OLD_ICONV=@OLD_ICONV@
 NO_DEFLATE_BOUND=@NO_DEFLATE_BOUND@
 FREAD_READS_DIRECTORIES=@FREAD_READS_DIRECTORIES@
 SNPRINTF_RETURNS_BOGUS=@SNPRINTF_RETURNS_BOGUS@
+PTHREAD_LIBS=@PTHREAD_LIBS@

configure.ac

@@ -479,6 +479,22 @@ AC_SUBST(NO_MKDTEMP)
 #
 # Define NO_SYMLINK_HEAD if you never want .git/HEAD to be a symbolic link.
 # Enable it on Windows.  By default, symrefs are still used.
+#
+# Define PTHREAD_LIBS to the linker flag used for Pthread support.
+AC_LANG_CONFTEST([AC_LANG_PROGRAM(
+  [[#include <pthread.h>]],
+  [[pthread_mutex_t test_mutex;]]
+)])
+${CC} -pthread conftest.c -o conftest.o > /dev/null 2>&1
+if test $? -eq 0;then
+ PTHREAD_LIBS="-pthread"
+else
+ ${CC} -lpthread conftest.c -o conftest.o > /dev/null 2>&1
+ if test $? -eq 0;then
+  PTHREAD_LIBS="-lpthread"
+ fi
+fi
+AC_SUBST(PTHREAD_LIBS)
 
 ## Site configuration (override autodetection)
 ## --with-PACKAGE[=ARG] and --without-PACKAGE