Refactor to use ExternalProgram for the command instead of duplicating
that code (badly). Also improve messages to say "or not executable"
when a script/command is not found.
Also allow ExternalPrograms to be passed as arguments to
run_command(). The only thing we're doing by preventing that is
forcing people to use prog.path()
And actually test that prog.path() works. The earlier test was just
running the command without checking if it succeeded.
Also make everything use prog.get_command() or get_path() instead of
accessing the internal member prog.fullpath directly.
Jussi Pakkanen
Nirbheek Chauhan