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

This implifies pathing in make/displayed output

MFC after:    3 weeks
Sponsored by: Dell EMC Isilon

lib/libsysdecode/Makefile

@@ -9,8 +9,8 @@ SRCS=	errno.c flags.c ioctl.c signal.c syscallnames.c utrace.c
 INCS=	sysdecode.h
 
 CFLAGS+= -I${.OBJDIR}
-CFLAGS+= -I${.CURDIR}/../../sys
-CFLAGS+= -I${.CURDIR}/../../libexec/rtld-elf
+CFLAGS+= -I${SRCTOP}/sys
+CFLAGS+= -I${SRCTOP}/libexec/rtld-elf
 
 MAN=	sysdecode.3 \
 	sysdecode_abi_to_freebsd_errno.3 \