Update gdb's configure instructions

gdb's README and configure instructions in the manual are quite out of
date.  They mention options that haven't existed for many years, among
other things.

This patch attempts to modernize the instructions somewhat.  It is not
exhaustive, just an improvement -- more could be done.  Perhaps even a
couple of the nodes should simply be removed and replaced by
references to the Autoconf manual.

The README file seems to have been generated from the Texinfo at some
point in the past.  I did not continue this, but instead edited it
separately.

gdb/ChangeLog
2018-09-29  Tom Tromey  <tom@tromey.com>

	* PROBLEMS: Rewrite.
	* README: Update.

gdb/doc/ChangeLog
2018-09-29  Tom Tromey  <tom@tromey.com>

	* gdb.texinfo (Requirements): Mention C++, GNU make.
	(Requirements): Sort the table.  Add more libraries.
	(Running Configure): Remove obsolete text.
	(Separate Objdir): Likewise.
	(Configure Options): Likewise.