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.

doc/Makefile

@@ -1,4 +1,5 @@
-#
+-include ../config.mk
+
 # Makefile to build the libxbps API documentation.
 #
 DOXYF	?=	xbps_api_doxyfile
@@ -22,3 +23,8 @@ doxygendocs: $(FILES)
 clean:
 	-rm -f images/*.$(FORMAT)
 	-rm -rf ../api
+
+.PHONY: install
+install:
+	install -d $(DESTDIR)$(SHAREDIR)/doc/xbps
+	cp -r ../api $(DESTDIR)$(SHAREDIR)/doc/xbps