Bring xditview back from the grave (part 2).

Fix things so it *really* won't compile if the environment isn't there.

This commit has been sitting in my to-do list for 2 months.  Xditview
should never have been half-removed, so now it's back with a vengance
because we want to keep groff intact as a package.

gnu/usr.bin/groff/Makefile

@@ -5,7 +5,7 @@ SUBDIR=		libgroff libdriver libbib\
 		refer lookbib indxbib lkbib \
 		tfmtodit addftinfo pfbtops psbb \
 		devX100 devX100-12 devX75 devX75-12 devascii devdvi devlatin1 \
-		devkoi8-r devps tmac mm man
+		devkoi8-r devps tmac mm man xditview
 
 # BSD already provides soelim
 MISC=		soelim

gnu/usr.bin/groff/xditview/Makefile

@@ -3,6 +3,7 @@
 .if exists(${DESTDIR}${X11BASE}/bin)
 .if exists(${DESTDIR}${X11BASE}/man/man1)
 .if exists(${DESTDIR}${X11BASE}/lib/X11/app-defaults)
+.if defined(X11_BUILD_AVAILABLE)
 
 BINDIR=		${X11BASE}/bin
 MANDIR=		${X11BASE}/man/man
@@ -30,6 +31,7 @@ afterinstall:
 .endif
 .endif
 .endif
+.endif
 
 .include "../Makefile.cfg"
 .include <bsd.prog.mk>