ninja: use clang by default on Mac, matching recent Xcodes

ninja: use cc and c++ instead of gcc and g++ by default on non-Windows.

See https://groups.google.com/forum/#!topic/gyp-developer/50IkuJBVGbU .

For Mac specifically:

g++ on my system is not Xcode's g++, but proper FSF GNU GCC g++. This
doesn't hurt Chromium, which sets clang as the compiler by default, but
does hurt projects that don't explicitly specify a compiler, including
the tests.

FSF GNU GCC g++ doesn't understand these options that xcode_emulation.py
adds:

g++: error: unrecognized command line option ‘-fasm-blocks’
g++: error: unrecognized command line option ‘-mpascal-strings’

R=thakis@chromium.org

Committed: https://code.google.com/p/gyp/source/detail?r=1791

[Nico, 2013-11-17 05:11:46]

Sure, why not. lgtm.

(I build my FSF gcc with --program-suffix=-4.8, so that "gcc" gives me whatever
apple ships as gcc these days (clang, as of xcode 5 / os x 10.9), and "gcc-4.8"
gives me fsf gcc. That way you don't run into issues like this.)

[Nico, 2013-11-17 10:11:39]

Actually! I just remembered the thread "Should the ninja generator default to cc
instead of gcc?" on gyp-dev – can we just use "cc" on all (non-win) platforms?
Then this will do the right thing on mac automatically and make things generally
simpler and better and whatnot.

[Mark Mentovai, 2013-11-17 15:21:29]

That works for me.

(Actually, FSF GNU GCC’s “make install” will give you its own copy of “c++,” but
I hate that and remove it.)

[Mark Mentovai, 2013-11-18 15:00:21]

Committed patchset #4 manually as r1791 (presubmit successful).