Make sure the 'use_system_*' variables are actually visible in the relevant gyp files.

third_party/libpng/libpng.gyp

Index: third_party/libpng/libpng.gyp
diff --git a/third_party/libpng/libpng.gyp b/third_party/libpng/libpng.gyp
index 6093d5f7c72d17d5a01cf35ae91c93bc797457af..1387b18c2af2a664c248b2af67da9b54ae0d6da5 100644
--- a/third_party/libpng/libpng.gyp
+++ b/third_party/libpng/libpng.gyp
@@ -4,7 +4,14 @@
 
 {
   'variables': {
-    'use_system_libpng%': 0,
+    'conditions': [
+      [ 'OS=="linux"', {
+        # Link to system .so since we already use it due to GTK.
+        'use_system_libpng%': 1,
+      }, {  # OS!="linux"
+        'use_system_libpng%': 0,
+      }],
+    ],
   },
   'conditions': [
     ['use_system_libpng==0', {