Copy configure option documentation to gdb.texinfo

I realized that while I'd updated the README, I didn't update
gdb.texinfo to document the options to configure.

This patch copies the text from README into gdb.texinfo, adding
Texinfo markup.

gdb/ChangeLog
2018-10-01  Tom Tromey  <tom@tromey.com>

	* README: Minor change.

gdb/doc/ChangeLog
2018-10-01  Tom Tromey  <tom@tromey.com>

	* gdb.texinfo (Configure Options): Document configure options.