Use SRCTOP-relative paths to other directories instead of .CURDIR-relative ones

This simplifies pathing in make/displayed output

MFC after:    3 weeks
Sponsored by: Dell EMC Isilon

lib/libiconv_modules/Makefile.inc

@@ -1,10 +1,10 @@
 # $FreeBSD$
 
-.PATH: ${.CURDIR}/../../libc/iconv
+.PATH: ${SRCTOP}/lib/libc/iconv
 
 SHLIB_MAJOR= 4
 WARNS?=	6
-CFLAGS+= -I${.CURDIR}/../../libc/iconv
+CFLAGS+= -I${SRCTOP}/lib/libc/iconv
 
 CFLAGS+=	-Dbool=_Bool
 

lib/libiconv_modules/mapper_parallel/Makefile

@@ -1,6 +1,6 @@
 # $FreeBSD$
 
-.PATH: ${.CURDIR}/../mapper_serial
+.PATH: ${.CURDIR:H}/mapper_serial
 
 SHLIB=	mapper_parallel
 SRCS+=	citrus_mapper_serial.c