Log Message:
mupdf: Minor fix/cleanups/patch hunks readd accidentally lost in 1.15.0 update

Fixes:
- Use `HAVE_GLUT ?= yes' instead of `HAVE_GLUT := yes' in patch-ab:
  in that way possible `MAKE_ENV+= HAVE_GLUT=no' in options.mk is honored
  (in 1.15.0 update this could lead to building and installing mupdf-gl
  and failing due PLIST mismatch despite `opengl' option was not
  set.)

Cleanups/patch hunks readds/improvements:
 - Avoid to remove non existent thirdparty/curl directory no longer exists,
   do not remove it in post-extract
 - Clarify why pkgsrc lcms2 could not be used
 - Remove curl leftover in buildlink3.mk
 - Update patch-Makethird, patch-ab, patch-ac comments to reflect reality
 - Populate SYS_GLUT_{CFLAGS,LIBS} via `pkg-config'