Swap 'make cleandir' and 'make includes' for real.

Should have been part of commitid uHRFWpwUBjIz0UuR.

Original message:

Move libcrypto, librpcsvc and gnu/usr.bin/cc/include from RDIRS to PRDIRS,
and add prereq targets, so some header files are generated by BUILDUSER
during 'make prereq' instead of by root during 'make includes'.

Switch the order of 'make cleandir' and 'make includes' during 'make build'
so we don't generate many files twice.

Except for some machine@ symlinks from ${MACHINE}/stand, /usr/obj is now
clean from files generated by root during 'make build'. Those will be
cleaned up in a second step.

help, testing & ok deraadt, input from natano, further testing rpe

Makefile

@@ -1,4 +1,4 @@
-#	$OpenBSD: Makefile,v 1.129 2016/10/06 18:56:17 natano Exp $
+#	$OpenBSD: Makefile,v 1.130 2016/10/18 02:47:07 tb Exp $
 
 #
 # For more information on building in tricky environments, please see
@@ -79,8 +79,8 @@ build:
 		false; \
 	fi
 	cd ${.CURDIR}/share/mk && exec ${MAKE} install
-	exec ${MAKE} includes
 	exec ${MAKE} cleandir
+	exec ${MAKE} includes
 	cd ${.CURDIR}/lib && \
 	    su ${BUILDUSER} -c '${MAKE} depend && exec ${MAKE}' && \
 	    NOMAN=1 exec ${MAKE} install