Don't ever build files depending on the directory where they are placed in.

It is obvious that its modification time will change with each such file
builded.
This bug cause whole libelf to rebuild itself each second make run
(and relink that files on each first make run) in the loop.

lib/libelf/Makefile

@@ -68,11 +68,9 @@ CLEANFILES=	${GENSRCS}
 CLEANDIRS=	sys
 CFLAGS+=	-I${.CURDIR} -I.
 
-sys/elf32.h sys/elf64.h sys/elf_common.h: sys
-	ln -sf ${.CURDIR}/../../sys/${.TARGET} ${.TARGET}
-
-sys:
+sys/elf32.h sys/elf64.h sys/elf_common.h: ${.CURDIR}/../../sys/${.TARGET}
 	mkdir -p ${.OBJDIR}/sys
+	ln -sf ${.CURDIR}/../../sys/${.TARGET} ${.TARGET}
 
 SHLIB_MAJOR=	1