DRTVWR-418: Attempt to make sense of Havok.cmake Linux logic.

Aside from crazy indentation, much of Havok.cmake is redundant testing of
DEBUG_PREBUILT and conditional MESSAGE(STATUS ...) output, not to mention
repeating stanzas for each of debug_dir, release_dir and relwithdebinfo_dir.

Use local functions and foreach() to try to manage redundancy so the details
of what it's actually trying to do don't get lost in the noise.