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

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
• Support for separate *.debug and *.exe binaries in DEBUG and RELWITHDEBINFO build types
  • SRB2_DEBUG_INSTALL -- Toggle *.debug binaries in install and packages
• SRB2_CPACK_GENERATOR -- Change type of package that CPACK will generate, e.g., ZIP, TGZ, 7Z.
• 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 build.

(Note that setting CCACHE_CP2=yes is not required because this MR defaults CCACHE to OFF. The build defaulted CCACHE to ON, which caused this requirement.)