Reimplement DJGPP's .gdbinit -> gdb.ini renaming.

This simplifies the .gdbinit filename selection logic.

We have a GDBINIT_FILENAME define that supposedly configurations would
override, but none do so.  Instead, the only configuration that wants
a different file name instead of ".gdbinit", djgpp, does a strcpy over
the gdbinit global array.  This means the array needs to be sized, and
the code that does that is doing the usual
'PATH_MAX/FILENAME_MAX/fallback constant/etc.' mess.

Instead of all that, it's much simpler to have configure specificy the
.gdbinit filename.  As bonus, we can then make the "gdbinit" global
array const.

gdb/
2013-07-01  Pedro Alves  <palves@redhat.com>

	* configure.ac (GDBINIT): Define, depending on host.
	* go32-nat.c (init_go32_ops): Don't override gdbinit here.
	* top.c (PATH_MAX): Delete fallback definition.
	(GDBINIT_FILENAME): Delete.
	(gdbinit): Reimplement as const char array set to the GDBINIT
	string constant.
	* top.h (gdbinit): Make const.

gdb/go32-nat.c

@@ -1028,9 +1028,6 @@ init_go32_ops (void)
 
   /* We are always processing GCC-compiled programs.  */
   processing_gcc_compilation = 2;
-
-  /* Override the default name of the GDB init file.  */
-  strcpy (gdbinit, "gdb.ini");
 }
 
 /* Return the current DOS codepage number.  */