Moved user options form Makefile to Makefile.conf

Hi.

I made Makefile more configurable, by moving user configuration options to optional Makefile.conf, for which there is a template: Makefile.conf.template.

Another thing is that I've changed test for commands availability from "if test -x COMMAND" to "if command -v COMMAND", which works not only for full paths, but for simple commands too.

I've also added a target to print build configuration options: make printconf
It shows which commands are available for build and used paths.