WITH_META_MODE: Lessen the filemon(4) requirement scope.

- Move the sys.mk filemon requirement to bsd.init.mk as a warning.
  This is intended only to show when building directly in a subdirectory
  without filemon loaded.
- Move the error into Makefile and only apply it when building
  from the META_TGT_WHITELIST target list.

-DNO_FILEMON can be used to suppress both the warning and the error but
makes WITH_META_MODE less useful.  It will only compare build commands
in this mode rather than track all dependencies.

This fixes installing from a jail which doesn't need filemon in this
phase [1].

Reported by:	Nikolai Lifanov <lifanov@mail.lifanov.com> [1]
Approved by:	re (implicit)
Sponsored by:	EMC / Isilon Storage Division

1 files changed, 7 insertions, 0 deletions

diff --git a/Makefile b/Makefile
index 5e2f951aa5796..b829916916dc8 100644
--- a/Makefile
+++ b/Makefile
@@ -219,6 +219,13 @@ _CAN_USE_META_MODE?= yes
 .if !defined(_CAN_USE_META_MODE)
 _MAKE+=	MK_META_MODE=no
 .unexport META_MODE
+.elif ${MK_META_MODE} == "yes"
+.if !exists(/dev/filemon) && !defined(NO_FILEMON) && !make(showconfig)
+# Require filemon be loaded to provide a working incremental build
+.error ${.newline}ERROR: The filemon module (/dev/filemon) is not loaded. \
+    ${.newline}ERROR: WITH_META_MODE is enabled but requires filemon for an incremental build. \
+    ${.newline}ERROR: 'kldload filemon' or pass -DNO_FILEMON to suppress this error.
+.endif	# !exists(/dev/filemon) && !defined(NO_FILEMON)
 .endif	# !defined(_CAN_USE_META_MODE)
 
 # Guess machine architecture from machine type, and vice versa.