Convert gnu to using SRCTOP

Prefer SRCTOP over CURDIR/../../contrib, etc. However, retain the "up
one level" instances of ../ because they are really relative to this
part of the tree and not a means to find the root of the tree. As
such, it's better to leave them since that further the goal of being
able to move directories if watned to in the future.

Differential Revision:	https://reviews.freebsd.org/D9932
Sponsored by:		Netflix
Silence	On:		arch@ (twice)

gnu/usr.bin/diff3/Makefile

@@ -1,9 +1,9 @@
 # $FreeBSD$
 
-DIFFSRC=${.CURDIR}/../../../contrib/diff/src
+DIFFSRC=${SRCTOP}/contrib/diff/src
 .PATH:  ${DIFFSRC} \
-	${.CURDIR}/../../../contrib/diff/lib \
-	${.CURDIR}/../../../contrib/diff/man
+	${SRCTOP}/contrib/diff/lib \
+	${SRCTOP}/contrib/diff/man
 
 PROG=		diff3
 SRCS=           diff3.c version-etc.c \
@@ -12,9 +12,9 @@ SRCS=           diff3.c version-etc.c \
 # Important for ctype macros!
 CFLAGS+=-funsigned-char
 
-CFLAGS+=-I${.CURDIR}/../../../contrib/diff/
-CFLAGS+=-I${.CURDIR}/../../../contrib/diff/src
-CFLAGS+=-I${.CURDIR}/../../../contrib/diff/lib
+CFLAGS+=-I${SRCTOP}/contrib/diff/
+CFLAGS+=-I${SRCTOP}/contrib/diff/src
+CFLAGS+=-I${SRCTOP}/contrib/diff/lib
 CFLAGS+=-DHAVE_CONFIG_H
 CFLAGS+=-DDEFAULT_DIFF_PROGRAM=\"/usr/bin/diff\"