CMake 3.0 compatibility:

CMAKE_CXX_COMPILER_ID may be "AppleClang" rather than "Clang" if it's the
separately versioned Apple variety on OSX. Make the test robust to either
by using MATCHES instead of STREQUAL.
New ShaderGroupBegin variant that takes a text description of a shader group

This is half the work to allow serialization of the set of Parameter, Shader,
and ConnectShaders calls that fully assemble a shader group.  Here is an
example:

    ShaderGroupBegin ("groupname", "surface",
                          "param float fin 3.14; " /*concatenate string*/
                          "shader a alayer;"
                          "shader b blayer;"
                          "connect alayer.f_out blayer.f_in;");

Add --group command to testshade so this can be tested and used from the
command line. Here's an example:

    testshade -v -group "param fin 3.14, shader a alayer, shader b blayer, connect alayer.f_out blayer.f_in"

(Commas are an allowed substitute for semicolons for statement separators,
and the last statement separator is optional if it's at the end of the
string; this makes it easier for command line use, where semicolons have
special meaning.)