CMAKE additions: SDL Mixer X, *.debug adjustments, CPACK adjustments, CCACHE

Repost of !991 (closed) for organization purposes. Code has not changed.

To support a Travis-CI overhaul that I'm doing, I've added the following features to CMAKE:

• Added SDL Mixer X support, Windows only.
• SRB2_ASSET_INSTALL -- Toggle assets in install and packages, so I can deploy non-asset packages.
• Support for separate *.debug and *.exe binaries in DEBUG and RELWITHDEBINFO build types
  • SRB2_DEBUG_INSTALL -- Toggle *.debug binaries in install and packages, so that I can exclude *.debug from installers.
• SRB2_CPACK_GENERATOR -- Change type of package that CPACK will generate, e.g., ZIP, TGZ, 7Z, so I can generate both archives and installers.
• SRB2_USE_CCACHE -- Force CCACHE usage. Not needed for Travis Linux jobs, but we do need this for OSX and Windows jobs.

You can see the results of this work in this non-deploying build.

See packages produced by this CMAKE here.