The FTP protocol is getting long in the tooth, and we should emphasize
HTTPS where that is available. This patch changes various gcc.gnu.org
URL's to instead use HTTPS.

For instance, kernel.org shut down FTP access in 2017, with the
explanation:

- The protocol is inefficient and requires adding awkward kludges to
  firewalls and load-balancing daemons
- FTP servers have no support for caching or accelerators, which has
  significant performance impacts
- Most software implementations have stagnated and see infrequent
  updates

ChangeLog:

2019-11-20  Janne Blomqvist  <jb@gcc.gnu.org>

        * configure.ac: Use https for gcc.gnu.org.
        * configure: Regenerated.

gcc/ChangeLog:

2019-11-20  Janne Blomqvist  <jb@gcc.gnu.org>

        * configure.ac: Use https for gcc.gnu.org.
        * configure: Regenerated.
        * doc/install.texi: Use https for gcc.gnu.org.
        * doc/sourcebuild.texi: Likewise.

gcc/testsuite/ChangeLog:

2019-11-20  Janne Blomqvist  <jb@gcc.gnu.org>

        * README: Use https for gcc.gnu.org.

libstdc++-v3/ChangeLog:

2019-11-20  Janne Blomqvist  <jb@gcc.gnu.org>

        * doc/html/api.html: Use https for gcc.gnu.org.
        * doc/xml/api.xml: Likewise.

maintainer-scripts/ChangeLog:

2019-11-20  Janne Blomqvist  <jb@gcc.gnu.org>

        * gcc_release: Use https for gcc.gnu.org.

From-SVN: r278526