docs: fix links pointing to gcc.gnu.org/install

Use https://gcc.gnu.org/onlinedocs/install/ instead.

ChangeLog:

	* configure.ac: Use new install URL.
	* configure: Regenerate.

gcc/ChangeLog:

	* Makefile.in: Use new install URL.
	* doc/gcc/gcc-command-options/machine-dependent-options/avr-options.rst:
	Use intersphinx link.
	* doc/gcc/gcc-command-options/options-to-control-diagnostic-messages-formatting.rst:
	Use new URL.
	* doc/gccint/source-tree-structure-and-build-system.rst: Use
	intersphinx link.
	* doc/install/host-target-specific-installation-notes-for-gcc.rst: Likewise.
	* doc/install/installing-gcc.rst: Likewise.