Fix broken URLs in libstdc++ API docs

	* doc/doxygen/mainpage.html: Fix broken URLs.

From-SVN: r252070

libstdc++-v3/ChangeLog

 2017-09-13  Jonathan Wakely  <jwakely@redhat.com>
 
+	* doc/doxygen/mainpage.html: Fix broken URLs.
+
 	PR libstdc++/81835
 	* doc/xml/manual/extensions.xml: Replace unstable URL.
 	* doc/html/manual/ext_demangling.html: Regenerate.

libstdc++-v3/doc/doxygen/mainpage.html

@@ -28,7 +28,7 @@
 
 <p>There are two types of documentation for libstdc++.  One is the
    distribution documentation, which can be read online
-   <a href="http://gcc.gnu.org/onlinedocs/libstdc++/index.html">here</a>
+   <a href="https://gcc.gnu.org/onlinedocs/libstdc++/index.html">here</a>
    or offline from the file doc/html/index.html in the library source
    directory.
 </p>
@@ -78,11 +78,11 @@
    pages.  See the section "Documentation Style"
    in <code>doc/xml/manual/appendix_contributing.xml</code> in the
    source tree for how to create (and write) the doxygen markup.
-  This style guide can also be viewed on the <a href="http://gcc.gnu.org/onlinedocs/libstdc++/manual/bk01apas04.html">web</a>.
+  This style guide can also be viewed on the <a href="https://gcc.gnu.org/onlinedocs/libstdc++/manual/documentation_hacking.html">web</a>.
 
 <h2>License, Copyright, and Other Lawyerly Verbosity</h2>
 <p>The libstdc++ documentation is released under
-   <a href="http://gcc.gnu.org/onlinedocs/libstdc++/manual/bk01pt01ch01s02.html">
+   <a href="https://gcc.gnu.org/onlinedocs/libstdc++/manual/appendix_gpl.html">
    these terms</a>.
 </p>
 <p>Part of the generated documentation involved comments and notes from