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>

 --- gcc-4.1.2.orig/boehm-gc/include/gc.h	2005-01-02 04:35:57.000000000 +0100

 +++ gcc-4.1.2/boehm-gc/include/gc.h	2007-03-04 23:33:07.000000000 +0100

 @@ -500,7 +500,7 @@

  #ifdef __linux__

  # include <features.h>

  # if (__GLIBC__ == 2 && __GLIBC_MINOR__ >= 1 || __GLIBC__ > 2) \

 -     && !defined(__ia64__)

 +     && !defined(__ia64__) && !defined(__UCLIBC__)

  #   ifndef GC_HAVE_BUILTIN_BACKTRACE

  #     define GC_HAVE_BUILTIN_BACKTRACE

  #   endif