Fix loading of lumps from directory on OS X

Use POSIX-compliant opendir() / readdir() functions instead fts_open() / fts_read()
Unlike Linux version, on OS X fts_read() inserts extra slash character between source directory and traversed entry paths
- alternative configuration using GLEW to get extension functions. This requires GL 2.0, though so it won't be promoted to the mainline for the time being.
- removed all pre GL 2.0 support.
Added GetActorPowerupTics ACS function
Merge branch 'master' of https://github.com/Edward850/zdoom
More editing flags

PSX-inspired flags to change the render styles of monsters (and other
things).
Less flags, more generic properties
Updated documentation
CANRAISE state definition

CANRAISE state for allowing a monster to be resurrected without an
infinite frame
Merge branch 'master' of https://github.com/rheit/zdoom
Merge branch 'master' of https://github.com/Edward850/zdoom
Merge branch 'master' of https://github.com/rheit/zdoom
Merge branch 'master' into GLEW_VERSION
- got rid of a lot of stuff that's no longer required with GLEW.
- replaced GLUs texture scaling with our own function. This is only used to scale down textures larger than what the hardware can handle so we do not need a dependency to an essentially deprecated library for it.
- fixed typo in xlat files (they don't use exactly C syntax.)
Init new non-0 fields for non-UDMF maps.
Hexen too.
Hexen also needs to init 0 fields.
Typo.
Merge branch 'master' of https://github.com/Gaerzi/zdoom
Merge branch 'master' of https://github.com/rheit/zdoom