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.

README

@@ -7,14 +7,15 @@ To build this you'll need:
   - openssl (development package with static libs)
   - libarchive >= 2.8.0 (development package with static libs)
 
-Optionally to build the API documentation (enabled with BUILD_API_DOCS
-make(1) argument):
+Optionally to build the API documentation:
 
   - graphviz and doxygen.
 
-By default it will be installed into /usr/local, can be changed
-by setting PREFIX and DESTDIR vars as make(1) arguments.
+Just run ./configure && make && make install. By default PREFIX is set
+to `/usr/local', can be changed by setting --prefix. The DESTDIR variable
+is also supported for package managers.
 
-Just run "make" and "make DESTDIR=~/xbps install".
+There are some more options that can be tweaked, see them with
+`./configure --help'.
 
 Good luck!