Change ninja rpath generation to be more like make.

Currently, the ninja generator adds -Wl,rpath in every executable for
linux targets, while the make generator includes it only for those
with .so dependencies. This patch changes the ninja generator to
behave like make.

Added a linux-specific unit test to verify the value and absence of
the rpath for executables with and without solib dependencies.

BUG=gyp:315
TEST=On linux: unit tests pass (45 no result), full
Chrome build no longer has rpath on chrome_sandbox.
Review URL: https://codereview.chromium.org/11821060/

Patch from Chris Wolfe <cwolfe@chromium.org>!



git-svn-id: http://gyp.googlecode.com/svn/trunk@1559 78cadc50-ecff-11dd-a971-7dbc132099af