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

src/include/Makefile

@@ -1,4 +1,4 @@
-#	$OpenBSD: Makefile,v 1.213 2016/09/23 09:18:05 sthen Exp $
+#	$OpenBSD: Makefile,v 1.214 2016/10/16 19:28:44 tb Exp $
 #	$NetBSD: Makefile,v 1.59 1996/05/15 21:36:43 jtc Exp $
 
 #	@(#)Makefile	5.45.1.1 (Berkeley) 5/6/91
@@ -29,14 +29,14 @@ LDIRS=	crypto ddb dev isofs miscfs msdosfs net netinet netinet6 \
 	netmpls net80211 nfs ntfs scsi sys ufs uvm
 
 # Places that need a prerequisite target met before includes
-PRDIRS=
+PRDIRS=	../lib/libcrypto ../lib/librpcsvc
 
 # Directories with an includes target
-RDIRS=	../lib/libcrypto ../lib/libcurses ../lib/libedit \
+RDIRS=	../lib/libcurses ../lib/libedit \
 	../lib/libevent ../lib/libexpat ../lib/libform \
 	../lib/libfuse ../lib/libkeynote ../lib/libmenu \
 	../lib/libossaudio ../lib/libpanel ../lib/libpcap \
-	../lib/libradius ../lib/librpcsvc ../lib/librthread \
+	../lib/libradius ../lib/librthread \
 	../lib/libskey ../lib/libssl  ../lib/libtls \
 	../lib/libusbhid ../lib/libutil ../lib/libz \
 	../usr.bin/lex ../gnu/lib/libreadline \
@@ -47,8 +47,7 @@ RDIRS+= ../gnu/usr.bin/gcc ../gnu/lib/libobjc
 PRDIRS+= ../gnu/lib/libstdc++
 .else
 RDIRS+= ../gnu/usr.bin/cc/libobjc
-PRDIRS+= ../gnu/lib/libstdc++-v3
-RDIRS+= ../gnu/usr.bin/cc/include
+PRDIRS+= ../gnu/lib/libstdc++-v3 ../gnu/usr.bin/cc/include
 .endif
 
 # prereq implies includes