The ports Makefile has this:

.if defined(WITH_JAIL)#{
	@if [ "`/sbin/mount | grep ^linprocfs`" = "" ]; \
	then \
		${ECHO_MSG} "ERROR: You must have LINPROCFS mounted before" ; \
		${ECHO_MSG} "starting to build the native JDK 1.5.0." ; \
		${ECHO_MSG} "" ; \
		${ECHO_MSG} "You may do it with the following commands:" ; \
		${ECHO_MSG} "" ; \
		${ECHO_MSG} "# kldload linprocfs" ; \
		${ECHO_MSG} "" ; \
		${ECHO_MSG} "and" ; \
		${ECHO_MSG} "" ; \
		${ECHO_MSG} "# mount -t linprocfs linprocfs /compat/linux/proc" ; \
		${ECHO_MSG} "" ; \
		exit 1 ; \
	fi
.endif#}

which as far as I can see, should be tested when the build is done
*outside* a jail. Inside a jail the port should check if the file
/compat/linux/proc/self exists (as a symlink).