Cherry-pick fix from gtest to fix compilation with libc++ standard library.
Get rid of hard-coded framework path in OpenGL detection.

Instead, rely on "-framework OpenGL" to make

#include <OpenGL/gl.h>
#include <OpenGL/glu.h>

work properly.
On OS X, automatically select a suitable sysroot based on available SDKs.

This provides OS X 10.9 users with a "no fiddling" default build setup. It may
cause pain to advanced builders. If a sysroot option is manually provided then
automatic detection is disabled.
Specify OS X frameworks using SCons FRAMEWORKS variable.