Add SRCTOP and RELDIR support.

This is a direct commit to stable/10.  It allows simpler MFCing from head where
these are being used more frequently.

Sponsored by:	EMC / Isilon Storage Division

share/mk/local.sys.mk

@@ -0,0 +1,13 @@
+# $FreeBSD$
+
+.if defined(.PARSEDIR)
+SRCTOP:= ${.PARSEDIR:tA:H:H}
+.else
+SRCTOP:= ${.MAKE.MAKEFILES:M*/local.sys.mk:H:H:H}
+.endif
+
+.if ${.CURDIR} == ${SRCTOP}
+RELDIR = .
+.elif ${.CURDIR:M${SRCTOP}/*}
+RELDIR := ${.CURDIR:S,${SRCTOP}/,,}
+.endif