Makefile: introduce PROG_NAME to contain the 'ct-ng' executable name
With the upcomming --program-{prefix,suffix,transform-name} options,
we'll have a need to centralise the actual executable name, that is
now no longer be a constant.
Rather than spread the prefix/suffix through-out the code, just
centralise the name setting in one place. Beside, transform-name
would not be possible without setting the name at a single location.
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>