Fix build problem (read-only ${PORTSDIR}) by adding

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.

sysutils/procmap/Makefile

@@ -12,15 +12,11 @@ DISTFILES=	#empty
 
 MAINTAINER=	jasone@freebsd.org
 
-NO_CHECKSUM=	yes
-NO_WRKSUBDIR=	yes
-
+# setting MAKEOBJDIR doesn't work for some reason
+MAKE_ENV=	MAKEOBJDIRPREFIX=${WRKDIR}
 WRKSRC=		${.CURDIR}/src
 
-do-fetch:
-	@${DO_NADA}
-
-pre-clean:
-	@$(SH) -ec "cd $(WRKSRC) ; make clean"
+pre-build:
+	@cd ${WRKSRC}; ${SETENV} ${MAKE_ENV} ${MAKE} obj
 
 .include <bsd.port.mk>