configure.ac: use $LIBS not $CFLAGS when testing -lpthread

Some linkers, namely the one on IRIX are rather strict concerning
the order or arguments for symbol resolution, i.e. no libraries
listed before objects or other libraries on the command line are
considered for symbol resolution.  Therefore, -lpthread can't work
if it's put in CFLAGS, because it will not be considered for
resolving pthread_key_create in conftest.o. Use $LIBS instead.

Signed-off-by: Rainer Canavan <git@canavan.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

configure.ac

@@ -1013,7 +1013,12 @@ elif test -z "$PTHREAD_CFLAGS"; then
   # would then trigger compiler warnings on every single file we compile.
   for opt in "" -mt -pthread -lpthread; do
      old_CFLAGS="$CFLAGS"
-     CFLAGS="$opt $CFLAGS"
+     old_LIBS="$LIBS"
+     case "$opt" in
+        -l*)  LIBS="$opt $LIBS" ;;
+        *)    CFLAGS="$opt $CFLAGS" ;;
+     esac
+
      AC_MSG_CHECKING([for POSIX Threads with '$opt'])
      AC_LINK_IFELSE([PTHREADTEST_SRC],
 	[AC_MSG_RESULT([yes])
@@ -1025,6 +1030,7 @@ elif test -z "$PTHREAD_CFLAGS"; then
 	],
 	[AC_MSG_RESULT([no])])
       CFLAGS="$old_CFLAGS"
+      LIBS="$old_LIBS"
   done
   if test $threads_found != yes; then
     AC_CHECK_LIB([pthread], [pthread_create],