MAKE_ENV=MAKEOBJDIRPREFIX=${WRKDIR} and a pre-build target to do a
"make obj". That obsoletes the pre-clean target since the generated
files all go into ${WRKDIR} and there's nothing to clean on the source
directory.
Also, fix several minor bogons. NO_CHECKSUM is a user variable, and
never should be set in a port Makefile. NO_WRKSUBDIR is not necessary
because WRKSRC is set explicitly. I don't know what the empty
do-fetch is doing there.
