multimedia/gavl: Update to 2.0.1

Project migrated to Github.

Remvove PNG option for simplicity as most consumers of this will have
ulitimately have graphics/png installed anyways.

Remove old cruft that doesn't apply to any supported version of FreeBSD
like CFLAGS+= -D_WITH_CPU_SET_T. We've had clock_nanosleep(2) since
11.1 too, so files/patch-gavl-time.c has been removed.