WITH_META_MODE: Enable printing of some of make's environment on error.

This will print a set of variables from make on error using
MAKE_PRINT_VAR_ON_ERROR.  It is already enabled for the DIRDEPS_BUILD.
It may make sense to enable this in the non-meta mode as well once
people are more used to its more verbose error output.

This makes it much simpler to see which .meta file is used when a
command files so that it may be inspected for the build command.

Suggested by:	sjg
Approved by:	re (implicit)
Sponsored by:	EMC / Isilon Storage Division

share/mk/local.sys.mk

@@ -1,6 +1,18 @@
 # $FreeBSD$
 
-.if ${MK_DIRDEPS_BUILD} == "yes"
+.if ${MK_DIRDEPS_BUILD} == "yes" || ${MK_META_MODE} == "yes"
+
+# Not in the below list as it may make sense for non-meta mode
+# eventually.  meta.sys.mk (DIRDEPS_BUILD) also already adds these in.
+.if ${MK_DIRDEPS_BUILD} == "no" && ${MK_META_MODE} == "yes"
+MAKE_PRINT_VAR_ON_ERROR += \
+	.ERROR_TARGET \
+	.ERROR_META_FILE \
+	.MAKE.LEVEL \
+	MAKEFILE \
+	.MAKE.MODE
+.endif
+
 MAKE_PRINT_VAR_ON_ERROR+= \
 	.CURDIR \
 	.MAKE \
@@ -12,7 +24,7 @@ MAKE_PRINT_VAR_ON_ERROR+= \
 	MACHINE_ARCH \
 	MAKEOBJDIRPREFIX \
 	MAKESYSPATH \
-	MAKE_VERSION\
+	MAKE_VERSION \
 	PATH \
 	SRCTOP \
 	OBJTOP \

tools/build/options/WITH_META_MODE

@@ -41,6 +41,9 @@ The meta files can also be useful for debugging.
 The build will hide commands ran unless
 .Va NO_SILENT
 is defined.
+Errors will cause
+.Xr make 1
+to show some of its environment for further debugging.
 .Pp
 The build operates as it normally would otherwise.
 This option originally invoked a different build system but that was renamed