META_MODE+filemon: Default -DNO_CLEAN enabled.

When using meta mode with filemon, the build is reliably incremental
safe.  Bmake will use the meta files, along with filemon information,
to rebuild targets when their dependencies change, commands change,
or files they generate are missing.

Sponsored by:	EMC / Isilon Storage Division

Makefile.inc1

@@ -144,6 +144,15 @@ CLEANDIR=	clean cleandepend
 CLEANDIR=	cleandir
 .endif
 
+.if ${MK_META_MODE} == "yes"
+# If filemon is used then we can rely on the build being incremental-safe.
+# The .meta files will also track the build command and rebuild should
+# it change.
+.if empty(.MAKE.MODE:Mnofilemon)
+NO_CLEAN=	t
+.endif
+.endif
+
 LOCAL_TOOL_DIRS?=
 PACKAGEDIR?=	${DESTDIR}/${DISTDIR}
 

share/man/man7/build.7

@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd March 29, 2016
+.Dd April 14, 2016
 .Dt BUILD 7
 .Os
 .Sh NAME
@@ -539,6 +539,14 @@ instead of
 .Dq make cleandir .
 .It Va NO_CLEAN
 If set, no object tree files are cleaned at all.
+This is the default when
+.Va WITH_META_MODE
+is used with
+.Xr filemon 4
+loaded.
+See
+.Xr src.conf 5
+for more details.
 Setting
 .Va NO_CLEAN
 implies