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

third_party/libjpeg/libjpeg.gyp

Index: third_party/libjpeg/libjpeg.gyp
diff --git a/third_party/libjpeg/libjpeg.gyp b/third_party/libjpeg/libjpeg.gyp
index c0259d65d006b6fd6b90d8a7b77c61325b7a7fae..1b0798bfc40196684c0389075629f39935f3b922 100644
--- a/third_party/libjpeg/libjpeg.gyp
+++ b/third_party/libjpeg/libjpeg.gyp
@@ -4,7 +4,14 @@
 
 {
   'variables': {
-    'use_system_libjpeg%': 0,
+    'conditions': [
+      [ 'OS=="linux"', {
+        # Link to system .so since we already use it due to GTK.
+        'use_system_libjpeg%': 1,
+      }, {  # OS!="linux"
+        'use_system_libjpeg%': 0,
+      }],
+    ],
   },
   'conditions': [
     ['use_system_libjpeg==0', {