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 gcc-4.2.1.orig/libstdc++-v3/configure gcc-4.2.1/libstdc++-v3/configure

 --- gcc-4.2.1.orig/libstdc++-v3/configure	2007-08-03 20:30:21.000000000 +0200

 +++ gcc-4.2.1/libstdc++-v3/configure	2007-08-03 20:33:56.000000000 +0200

 @@ -7514,6 +7514,9 @@

  cat >>conftest.$ac_ext <<_ACEOF

  /* end confdefs.h.  */

  #include <complex.h>

 +#ifdef __UCLIBC__

 +#error ugly hack to make sure configure test fails here for cross until uClibc supports the complex funcs

 +#endif

  int

  main ()

  {