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/librt/Makefile

@@ -5,7 +5,7 @@
 PACKAGE=lib${LIB}
 LIB=rt
 SHLIB_MAJOR= 1
-CFLAGS+=-I${.CURDIR}/../libc/include -I${.CURDIR}
+CFLAGS+=-I${SRCTOP}/lib/libc/include -I${.CURDIR}
 .ifndef NO_THREAD_STACK_UNWIND
 CFLAGS+=-fexceptions
 .endif
@@ -18,7 +18,7 @@ SRCS+= aio.c mq.c sigev_thread.c timer.c
 
 PRECIOUSLIB=
 
-VERSION_DEF=${.CURDIR}/../libc/Versions.def
+VERSION_DEF=${SRCTOP}/lib/libc/Versions.def
 SYMBOL_MAPS=${.CURDIR}/Symbol.map
 
 .if ${MK_TESTS} != "no"