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>

 diff -durN binutils-2.20.1.orig/libiberty/Makefile.in binutils-2.20.1/libiberty/Makefile.in

 --- binutils-2.20.1.orig/libiberty/Makefile.in	2009-08-23 21:03:58.000000000 +0200

 +++ binutils-2.20.1/libiberty/Makefile.in	2010-08-17 19:32:19.000000000 +0200

 @@ -227,6 +227,7 @@

  	  $(AR) $(AR_FLAGS) $(TARGETLIB) \

  	    $(REQUIRED_OFILES) $(EXTRA_OFILES) $(LIBOBJS); \

  	  $(RANLIB) $(TARGETLIB); \

 +	  cp $(TARGETLIB) ../ ; \

  	  cd ..; \

  	else true; fi