libstdc++: Use AS_IF in configure.ac

This ensures that anything that depends on AC_REQUIRE is hoisted out of
the conditional block.

The always-false test x"long_double_math_on_this_cpu" = x"yes" condition
is not altered by this commit, only changed to use the AS_IF syntax.

libstdc++-v3/ChangeLog:

	* configure.ac: Use AS_IF.
	* configure: Regenerate.