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.

include/Makefile

@@ -1,4 +1,4 @@
-include ../vars.mk
+-include ../config.mk
 
 INCS =	xbps_api.h
 

include/strlcat.h

@@ -0,0 +1,8 @@
+#ifndef STRLCAT_H
+#define STRLCAT_H
+
+#include <sys/types.h>
+
+size_t strlcat(char *, const char *, size_t);
+
+#endif

include/strlcpy.h

@@ -0,0 +1,8 @@
+#ifndef STRLCPY_H
+#define STRLCPY_H
+
+#include <sys/types.h>
+
+size_t strlcpy(char *, const char *, size_t);
+
+#endif