This patch uses the support for pretty-printing escaped URLs added in
the previous patch (PR 87488) so that in a sufficiently capable terminal
the [-Wname-of-option] emitted at the end of each diagnostic becomes a
hyperlink to the documentation for that option on the GCC website.

I've tested it with Fedora 30's GNOME Terminal (3.32.2 with VTE 0.56.3);
the option text is printed with a dotted underline, hovering shows the
URL, and on right-clicking it offers menu items to visit/copy the URL.

gcc/ChangeLog:
	PR 87488
	* Makefile.in (CFLAGS-opts.o): Pass in DOCUMENTATION_ROOT_URL via
	-D.
	* configure.ac (--with-documentation-root-url): New option.
	* configure: Regenerate.
	* diagnostic-format-json.cc (json_end_diagnostic): If there is an
	option URL, add it as a new string field of the diagnostic option.
	* diagnostic.c (diagnostic_initialize): Initialize get_option_url.
	(print_option_information): If get_option_url is non-NULL, call
	it, and if the result is non-NULL, potentially emit an escape
	sequence to markup the option text with the resulting URL.
	* diagnostic.h (diagnostic_context::get_option_url): New callback.
	* doc/invoke.texi (-fdiagnostics-format=): Add "option_url" to
	example of JSON output.
	* opts-diagnostic.h (get_option_url): New decl.
	* opts.c (get_option_url): New function.
	* toplev.c (general_init): Initialize the get_option_url callback.

gcc/testsuite/ChangeLog:
	PR 87488
	* c-c++-common/diagnostic-format-json-2.c: Expect an "option_url"
	field.
	* c-c++-common/diagnostic-format-json-3.c: Likewise.
	* gfortran.dg/diagnostic-format-json-2.F90: Likewise.
	* gfortran.dg/diagnostic-format-json-3.F90: Likewise.
	* jit.dg/test-error-array-bounds.c (create_code): Ensure that
	error messages don't contain escaped URLs.

From-SVN: r276843