re PR bootstrap/46810 (sparc64-linux bootstrap fails with "C++ preprocessor "/lib/cpp" fails sanity check")

	PR bootstrap/46810
	* configure.ac: Disable AC_MSG_ERROR while looking for the C++
	preprocessor.
	* configure: Rebuild.

Co-Authored-By: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>

From-SVN: r167572