configure: always set SPHINX_BUILD

During the Sphinx-migration development, I used
SPHINX_BUILD='' in order to skip building info and manual
pages in gcc folder. However, we've got HAS_SPHINX_BUILD
which is the correct flag for that.

With the patch, one will get a nicer error message when
sphinx-build is missing and one builds (explicitly) a target which
depends on it.

	PR other/107620

gcc/ChangeLog:

	* configure: Regenerate.
	* configure.ac: Always set sphinx-build.

libgomp/ChangeLog:

	* configure: Regenerate.
	* configure.ac: Always set sphinx-build.

libiberty/ChangeLog:

	* configure: Regenerate.
	* configure.ac: Always set sphinx-build.

libitm/ChangeLog:

	* configure: Regenerate.
	* configure.ac: Always set sphinx-build.

libquadmath/ChangeLog:

	* configure: Regenerate.
	* configure.ac: Always set sphinx-build.