Added a configure script to emulate GNU autoconf and related changes.

Changes included in this set:

 * Added strlcat() and strlcpy() from OpenBSD, always use them if the
   system does not have them built in.
 * Changed an array of PATH_MAX size allocated in the stack, to a dynamically
   allocated buffer from heap. This should reduce memory usage a bit.
 * Simplify code that implemented a homegrown realpath(3) implementation,
   simply use realpath(3).
 * If compiler supports -fstack-protector, build all code with
   -D_FORTIFY_SOURCE=2 and --param ssp-buffer-size=1 so that all
   buffers are protected.

bin/xbps-repo/Makefile

@@ -1,5 +1,5 @@
 TOPDIR = ../..
-include $(TOPDIR)/vars.mk
+-include $(TOPDIR)/config.mk
 
 BIN =	xbps-repo
 OBJS =	main.o util.o index.o repository.o