lib9: pass $PLAN9_TARGET via CPP for get9root fallback value

Allows this function to always return the proper path in situations
where the $PLAN9 environment variable is not set, i.e. a rc login shell.

src/lib9/get9root.c

@@ -11,7 +11,6 @@ get9root(void)
 
 	if((s = getenv("PLAN9")) != 0)
 		return s;
-	/* could do better - search $PATH */
-	s = "/usr/local/plan9";
+	s = PLAN9_TARGET;
 	return s;
 }

src/lib9/mkfile

@@ -175,6 +175,9 @@ HFILES=\
 %.$O: utf/%.c
 	$CC $CFLAGS utf/$stem.c
 
+get9root.$O: get9root.c
+	$CC $CFLAGS -DPLAN9_TARGET=\"$PLAN9_TARGET\" get9root.c
+
 XLIB=$PLAN9/lib/$LIB
 
 testfmt: testfmt.$O $XLIB

src/mkmk.sh

@@ -36,7 +36,7 @@ echo cd `pwd`
 9c  exitcode.c
 9c  fcallfmt.c
 9c  frand.c
-9c  get9root.c
+9c  -DPLAN9_TARGET=\"$PLAN9_TARGET\" get9root.c
 9c  getcallerpc.c
 9c  getenv.c
 9c  getfields.c