Use SRCTOP to refer to awk source in contrib/awk and remove unnecessary AWKSRC prefix

for maketab.c

The former simplifies pathing in make/displayed output, whereas the latter was just
unnecessarily superfluous since .PATH referenced the path to maketab.c earlier on in
the Makefile.

MFC after:	1 week
Sponsored by:	Dell EMC Isilon

usr.bin/awk/Makefile

@@ -1,6 +1,6 @@
 # $FreeBSD$
 
-AWKSRC=	${.CURDIR}/../../contrib/one-true-awk
+AWKSRC=	${SRCTOP}/contrib/one-true-awk
 .PATH: ${AWKSRC}
 
 PROG=	awk
@@ -26,6 +26,6 @@ proctab.c: maketab
 	${BTOOLSPATH:U.}/maketab > proctab.c
 
 build-tools: maketab
-maketab: ytab.h ${AWKSRC}/maketab.c ${BUILD_TOOLS_META}
+maketab: ytab.h maketab.c ${BUILD_TOOLS_META}
 
 .include <bsd.prog.mk>