normalize perl shebangs

don't require perl to be in a fixed location

Change-Id: Icc8b6113a2a3626f847fe46409334a03f1db5c85
diff --git a/build/make/ads2armasm_ms.pl b/build/make/ads2armasm_ms.pl
index 1def539..95c8084 100755
--- a/build/make/ads2armasm_ms.pl
+++ b/build/make/ads2armasm_ms.pl
@@ -1,4 +1,4 @@
-#!/usr/bin/perl
+#!/usr/bin/env perl
 ##
 ##  Copyright (c) 2013 The WebM project authors. All Rights Reserved.
 ##
diff --git a/build/make/thumb.pm b/build/make/thumb.pm
index d8d04aa..9604c8e 100644
--- a/build/make/thumb.pm
+++ b/build/make/thumb.pm
@@ -1,4 +1,4 @@
-#!/usr/bin/perl
+#!/usr/bin/env perl
 ##
 ##  Copyright (c) 2013 The WebM project authors. All Rights Reserved.
 ##