>   , CHANGES_URL ("gcc-10/changes.html#empty_base");
>
> where the macro would just use preprocessor string concatenation?

Ok, the following patch implements it (doesn't introduce a separate
macro and just uses CHANGES_ROOT_URL "gcc-10/changes.html#empty_base"),
in addition adds the documentation Joseph requested.

2020-04-30  Jakub Jelinek  <jakub@redhat.com>

	* configure.ac (--with-documentation-root-url,
	--with-changes-root-url): Diagnose URL not ending with /,
	use AC_DEFINE_UNQUOTED instead of AC_SUBST.
	* opts.h (get_changes_url): Remove.
	* opts.c (get_changes_url): Remove.
	* Makefile.in (CFLAGS-opts.o): Don't add -DDOCUMENTATION_ROOT_URL
	or -DCHANGES_ROOT_URL.
	* doc/install.texi (--with-documentation-root-url,
	--with-changes-root-url): Document.
	* config/arm/arm.c (aapcs_vfp_is_call_or_return_candidate): Don't call
	get_changes_url and free, change url variable type to const char * and
	set it to CHANGES_ROOT_URL "gcc-10/changes.html#empty_base".
	* config/s390/s390.c (s390_function_arg_vector,
	s390_function_arg_float): Likewise.
	* config/aarch64/aarch64.c (aarch64_vfp_is_call_or_return_candidate):
	Likewise.
	* config/rs6000/rs6000-call.c (rs6000_discover_homogeneous_aggregate):
	Likewise.
	* config.in: Regenerate.
	* configure: Regenerate.