Add --enable-codesign to gdb's configure

macOS requires that the gdb executable be signed in order to be able
to successfully use ptrace.  This must be done after each link.

This patch adds a new --enable-codesign configure option so that this
step can be automated.

gdb/ChangeLog
2018-06-28  Tom Tromey  <tom@tromey.com>

	* NEWS: Mention --enable-codesign.
	* silent-rules.mk (ECHO_SIGN): New variable.
	* configure.ac: Add --enable-codesign.
	* configure: Rebuild.
	* Makefile.in (CODESIGN, CODESIGN_CERT): New variables.
	(gdb$(EXEEXT)): Optionally invoke codesign.