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>

 http://git.frugalware.org/gitweb/gitweb.cgi?p=frugalware-current.git;a=blob_plain;f=source/base/glibc/make-3.82-fix.patch;hb=8217c32ecc2e14962847ba3d8a272eb64a3dba4f

 --- glibc-2.10.1.OLD/manual/Makefile

 +++ glibc-2.10.1.NEW/manual/Makefile

 @@ -232,7 +232,9 @@

  .PHONY: stubs

  stubs: $(objpfx)stubs

  endif

 -$(objpfx)stubs ../po/manual.pot $(objpfx)stamp%:

 +$(objpfx)stubs ../po/manual.pot:

 +	touch $@

 +$(objpfx)stamp%:

  	$(make-target-directory)

  	touch $@