Improve documentation of GDB build requirements and options

MPFR is now mandatory, so its previous description in Requirements
was inappropriate and out of place.  In addition, the description
of how to go about specifying 'configure' time options for
building with libraries was highly repetitive.  Some of the text
was also outdated and used wrong markup.

Original patch and suggestions from Philippe Blain
<levraiphilippeblain@gmail.com>.

ChangeLog:
2023-04-05  Eli Zaretskii  <eliz@gnu.org>

	* gdb/doc/gdb.texinfo (Requirements, Configure Options): Update and
	rearrange; improve and fix markup.