gdb/Makefile: rewrite dependencies for config.status target

I noticed something weird, the rule for the config.status target looks
like this:

  config.status: $(srcdir)/configure configure.nat configure.tgt configure.host ../bfd/development.sh
          $(SHELL) config.status --recheck

What bothered me is that 'configure' is specified as being in
$(srcdir), while all of the other files are not, even though those
files are in the same $(srcdir) as the configure script.

However, I tried touching one of those files, and the config.status
rule does trigger!

This is thanks to the VPATH variable, which is set to $(srcdir), so
make looks in $(srcdir) for any dependencies.

However, this inconsistency bothers me.  Better, I think, to add the
$(srcdir) prefix to each of these files.

I also spotted that the configure script also includes the files
../bfd/config.bfd, yet that is missing from the include list, so in
this commit I plan to add this as a dependency.

The configure script also pulls in two TCL and TK related files:

	. ${TCL_BIN_DIR}/tclConfig.sh
	. ${TK_BIN_DIR}/tkConfig.sh

However, I don't think ${TCL_BIN_DIR} and ${TK_BIN_DIR} are currently
visible in GDB's Makefile, so I'm not planning to add these
dependencies at this time.

In this commit I add a new variable config_status_deps which holds the
list of all the dependencies for config.status, with the $(srcdir)
prefix included, and then I use this in the config.status rule.

After this commit config.status will regenerate if config.bfd changes,
which it wouldn't before, but nothing else changes.

Approved-By: Simon Marchi <simon.marchi@efficios.com>