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.

prog.mk

@@ -1,5 +1,7 @@
+-include $(TOPDIR)/config.mk
+
 OBJS	?= main.o
-CFLAGS  += -fPIE
+CFLAGS  += $(PROG_CFLAGS)
 LDFLAGS += -lxbps
 
 .PHONY: all
@@ -39,5 +41,5 @@ $(BIN).static: $(OBJS)
 
 $(BIN): $(OBJS)
 	@printf " [CCLD]\t\t$@\n"
-	@$(CC) $^ $(CPPFLAGS) $(CFLAGS) $(LDFLAGS) -pie -o $@
+	@$(CC) $^ $(CPPFLAGS) $(CFLAGS) $(LDFLAGS) $(PROG_LDFLAGS) -o $@