added #include <cstdlib> to glsl_optimizer_tests.cpp to fix:

 tests/glsl_optimizer_tests.cpp:324:180: error: ‘system’ was not declared in this scope
  int res = system("/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/usr/bin/metal metalTemp.metal -o metalTemp.o -std=ios-metal1.0 -Wno-parentheses-equality");

This was happening with cmake on both Linux (gcc) and OSX (clang),
so perhaps it is something that is taken care of by Xcode.

removed the empty definition in standalone_scaffolding.h (but kept the declaration)
and added specific definitions to the executables that need it (glslopt and glsl_test)

This multiple definition was occurring in Linux and OSX with cmake.  The changes
build properly on Linux-cmake, OSX-cmake, and OSX-xcode. The tests pass on both
OSX-builds.  The Linux-cmake tests fail, but they fail before this commit.
I do not have access to MSVC.

For some reason gcc (versions 4.6 and 4.8) optimizes out the
"Trim out variables" loop in `ir_array_reference_visitor::get_split_list`,
even we don't want it too.

If that loop disappears, then the variables are never considered split
and not removed from the list, so the optimization continues
indefinitely.

Placing a memory fence in the loop prevents it from being optimized
out; this is done just with gcc.