Add install and uninstall targets

The build system now support installing, supporting both $(prefix) and
$(DESTDIR) variables. A few helper functions are added to ease installation
and the $I variable is provided as the base directory where stuff should
be installed (i.e. $(DESTDIR)$(prefix)). The $(prefix) is passed to
compiled programs as a pre-processor define (PREFIX) and targets are
re-built when it changes.

Programs should "register" files to install adding them to the $(install)
variable (similar as they register targets to compile in the $(all)
variable).

A few common rules are included in Lib.mak to install binaries and
libraries. To install other type of files the rule should be provided by
the makefile writer (but a function is provided to make this easy).

The test project is updated to install binaries, libraries and headers.