Mac cloud print proxy work.

Turn on use_cups for the Mac, add in the required library, and tweak the code
to get it linking.  There is still more work to do; this is just a gyp-oriented
change to get things started.

BUG=none
TEST=All targets still build on all platforms.

Review URL: http://codereview.chromium.org/4167006

git-svn-id: http://src.chromium.org/svn/trunk/src/build@64441 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

common.gypi

@@ -392,7 +392,7 @@
           'NACL_WIN64',
         ],
       }],
-      ['OS=="linux" and chromeos==0 and target_arch!="arm"', {
+      ['OS=="mac" or (OS=="linux" and chromeos==0 and target_arch!="arm")', {
         'use_cups%': 1,
       }, {
         'use_cups%': 0,