CMake Macro Improvements

Some improvements to the CMake macros provided by Rez.

* Colorize - based on an answer found on Stackoverflow, colorize the `message` output of from CMake. (see attached image for an example).

* Doxygen - fix a problem with the Doxygen macro failing due to a missing template Doxyfile.  I've added it to a `template_files` folder that sits alongside `cmake_files`.  In this way it's available to all build systems should they require it, and out of the Rez root where it lived previously.

* Doxygen - declare the doxyfile as a dependency of the custom cmmand so CMake is aware when it has changed (this is useful when building without running `make clean` first).

* System INCLUDE_DIRS - allow an INCLUDE_DIRS to be defined as SYSTEM, thereby removing them from compiler warnings.