Properly handle parallel make ("make -j"), and only rerun gyp if necessary.

Properly handle parallel make ("make -j"), and only rerun gyp if necessary.

This should completely resolve http://code.google.com/p/skia/issues/detail?id=330 ('building multiple targets in parallel fails on Linux')

This should resolve http://code.google.com/p/skia/issues/detail?id=313 ('toplevel makefile always reruns gyp_skia') for Linux, but not Mac.  (See details in comments within the file)
Review URL: http://codereview.appspot.com/4827043

git-svn-id: http://skia.googlecode.com/svn/trunk@1968 2bbb7eff-a529-9590-31e7-b0007b416f81