xcode_emulation: fix on CLT-only systems redux

This reimplements r1819 in a way that should avoid breaking Xcode, and
should also allow setting framework paths on CLT-only systems.

BUG=code.google.com/p/gyp/issues/detail?id=292
Review URL: https://codereview.chromium.org/118473009/
Patch from mistydemeo@gmail.com!

Review URL: https://codereview.chromium.org/117013003

git-svn-id: http://gyp.googlecode.com/svn/trunk@1825 78cadc50-ecff-11dd-a971-7dbc132099af