add toplevel Makefile that works for Mac and Unix

allows developers to use these exact same build commands on Mac and Linux:

#   # Clean everything
#   make clean
#
#   # Build and run tests (in Debug mode)
#   make tests
#   out/Debug/tests
#
#   # Build and run tests (in Release mode)
#   make tests BUILDTYPE=Release
#   out/Release/tests
#
#   # Build bench and SampleApp (both in Release mode), and then run them
#   make SampleApp bench BUILDTYPE=Release
#   out/Release/bench -repeat 2
#   out/Release/SampleApp
#
#   # Build all targets (in Debug mode)
#   make

[reed1, 2011-06-22 19:12:28]

I am the mostest ignorant person on how Makefiles work, so if we can find
another reviewer (now or in the future) that would be good.

That said... LGTM

[epoger, 2011-06-22 19:17:52]

committed as r1675