ninja/make (Xcode emulation): permit SDKROOT to be an absolute path (again)

This broke in r1595. The old logic was to compute a path for anything that
began with "macosx". The broken logic computed a path for everything. The new
logic computes a path unless it begins with "/".
Review URL: https://codereview.chromium.org/13866019

git-svn-id: http://gyp.googlecode.com/svn/trunk@1606 78cadc50-ecff-11dd-a971-7dbc132099af