Use ::isinf and ::isnan if libc defines them

Use ::isinf and ::isnan if libc defines them

	PR libstdc++/48891
	* acinclude.m4 (GLIBCXX_CHECK_MATH11_PROTO): Check for obsolete isinf
	and isnan functions.
	* config.h.in: Regenerate.
	* configure: Regenerate.
	* include/c_global/cmath (isinf(double), isnan(double))
	[_GLIBCXX_HAVE_OBSOLETE_ISINF_ISNAN]: Import via using-directive.
	* testsuite/26_numerics/headers/cmath/48891.cc: New.

From-SVN: r232327