Support for environment variables in custom commands at build time

Here's a first stab at support for per-target environment variables for custom targets / commands. For more information, see ticket on the kitware bug tracker here: http://public.kitware.com/Bug/view.php?id=5145

You can see a usage example [here](https://github.com/jbohren-forks/CMake/blob/master/Tests/CustomCommandEnvVariables/CMakeLists.txt).

You can run a corresponding unit test from the main directory by executing:

    ctest -R CustomCommandEnvVariables

Note that support is implemented for:
- UNIX Makefiles
- XCode Projects
- Visual Studio Projects
- Ninja