Merge commit 'osx-fixes' into unification/master

SDL on OS X does not include pkg-config files, so assume the user put it
in the OS X specific "framework" directory.

Reported by btb: https://github.com/dxx-rebirth/dxx-rebirth/issues/14
Requested by btb: https://github.com/dxx-rebirth/dxx-rebirth/issues/15
Search for SDL_mixer pkg-config too

Users might install SDL and SDL_mixer to separate paths, so search for
SDL_mixer.pc too.
Merge pull request #16 from btb/osx-issues

Osx issues
Combine pkg-config caches
Fix escaping empty flags
Rewrite gr_linear_rep_movsdm with std::transform

Eliminate deprecated register.
Refactor gr_ubitmap012
Pack listbox better
Use range_for in light rendering
Rewrite scale_line to use std::fill

Eliminate inline byte-by-byte store.
Fix some -Wsign-compare warnings
Use pkg-config on Darwin if available