Add indentation offset guess for gyp emacs mode.

This fixes the indentation for some of the gyp files using 4 spaces
indentation instead of the usual 2.
Also adds "ldflags" in the list of keywords.

Review URL: https://codereview.chromium.org/598023002/
Patch from Fabrice de Gans <fdegans@chromium.org>!


git-svn-id: http://gyp.googlecode.com/svn/trunk@1995 78cadc50-ecff-11dd-a971-7dbc132099af