Honor LDFLAGS on Mac and Win. 

BUG=309412
Review URL: https://codereview.chromium.org/30813002/

Patch from James Kozianski <koz@chromium.org>!



git-svn-id: http://gyp.googlecode.com/svn/trunk@1765 78cadc50-ecff-11dd-a971-7dbc132099af