Search for SDL_mixer pkg-config too

Users might install SDL and SDL_mixer to separate paths, so search for
SDL_mixer.pc too.
use correct path for bundle executable
use correct path for localized strings
Simplify by using generic Info.plist, builtin substitions
<string> required for std::string
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
reenable int3
don't know why int3 would be here
check for GM_MULTI before trying to do network jobs