graphics/gts: Unbreak build WITH=NETPBM

happrox.c:24:10: fatal error: 'pgm.h' file not found

The netpbm update from r451378 moved netpbm headers from include/ to
include/netpbm/, but gts looks for them in include/ only.

While here also clean up the port a little bit:

• Remove NETPBM_DESC since it's already in bsd.options.desc.mk
• Disable netpbm via an autoconf variable instead of patching configure

PR: 222958
Reported by: Phil Pennock <freebsd@phil.spodhuis.org>
Approved by: erik@bz.bzflag.bz (maintainer timeout, 2 weeks)