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

third_party/libxslt/libxslt.gyp

Index: third_party/libxslt/libxslt.gyp
diff --git a/third_party/libxslt/libxslt.gyp b/third_party/libxslt/libxslt.gyp
index 1f6281072facc1f8c69c055db311cb29ecd2d5f6..4a8f81216dadc069552d3ba74af6cb4a70ab63e2 100644
--- a/third_party/libxslt/libxslt.gyp
+++ b/third_party/libxslt/libxslt.gyp
@@ -8,8 +8,13 @@
       ['OS=="linux" or OS=="freebsd"', {'os_include': 'linux'}],
       ['OS=="mac"', {'os_include': 'mac'}],
       ['OS=="win"', {'os_include': 'win32'}],
+      [ 'OS=="linux"', {
+        # Link to system .so since we already use it due to GTK.
+        'use_system_libxslt%': 1,
+      }, {  # OS!="linux"
+        'use_system_libxslt%': 0,
+      }],
     ],
-    'use_system_libxslt%': 0,
   },
   'targets': [
     {