Improve make pthread linking mechanism

Signed-off-by: Paul Elliott <paul.elliott@arm.com>
diff --git a/tests/scripts/all.sh b/tests/scripts/all.sh
index 65203e8..933c563 100755
--- a/tests/scripts/all.sh
+++ b/tests/scripts/all.sh
@@ -218,7 +218,7 @@
     done
 
     # Option to enable linking with pthreads under make
-    MAKE_THREADING_FLAGS="PTHREAD=1"
+    MAKE_THREADING_FLAGS="THREADING=pthread"
 }
 
 # Test whether the component $1 is included in the command line patterns.