Use LIBXOSRC instead of LIBXO when defining the path to contrib/libxo

The latter is already defined in bsd.libnames.mk, so avoid the conflict
in case someone copy-pastes make variables

While here, switch path to the top of the source tree with SRCTOP

lib/libxo/Makefile

@@ -1,8 +1,8 @@
 # $FreeBSD$
 
-LIBXO=	${.CURDIR:H:H}/contrib/libxo
+LIBXOSRC=	${SRCTOP}/contrib/libxo
 
-.PATH:	${LIBXO}/libxo
+.PATH:	${LIBXOSRC}/libxo
 
 LIB=	xo
 SHLIB_MAJOR=0
@@ -11,7 +11,7 @@ SHLIBDIR?=      /lib
 
 SRCS=	libxo.c xo_encoder.c xo_syslog.c
 
-CFLAGS+=-I${LIBXO}/libxo
+CFLAGS+=-I${LIBXOSRC}/libxo
 CFLAGS+=-DXO_ENCODERDIR=\"/usr/lib/libxo/encoder\"
 
 INCS=	xo.h xo_encoder.h

usr.bin/xo/Makefile

@@ -1,14 +1,14 @@
 # $FreeBSD$
 
-LIBXO=	${.CURDIR:H:H}/contrib/libxo
+LIBXOSRC=	${SRCTOP}/contrib/libxo
 
-.PATH:	${LIBXO}/xo
+.PATH:	${LIBXOSRC}/xo
 
 PROG=	xo
 MAN=	xo.1
 
 # XXX For xoversion.h
-CFLAGS+=-I${LIBXO}/libxo
+CFLAGS+=-I${LIBXOSRC}/libxo
 
 LIBADD=	xo util