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

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index 0f42c376a77a91accd066bc96d1342dd08eea734..ece3401a7dff161fcb45027c6d1fb8283e740373 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -44,20 +44,6 @@
           # hosts should pass an explicit target_arch to gyp.
           'target_arch%':
             '<!(uname -m | sed -e "s/i.86/ia32/;s/x86_64/x64/;s/arm.*/arm/")',
-
-          # On Linux, we use the system versions of several libraries. We end
-          # up pulling these in as .so's anyway since they are already in our
-          # transitive closure due to GTK.
-          'use_system_zlib': 1,
-          'use_system_libjpeg': 1,
-          'use_system_bzip2': 1,
-          'use_system_libpng': 1,
-          'use_system_libjpeg': 1,
-          'use_system_libxslt': 1,
-
-          # We use our own copy of libssl, although we still need to link
-          # against the rest of NSS.
-          'use_system_ssl': 0,
         }, {  # OS!="linux"
           'target_arch%': 'ia32',
         }],