long double and GLIBCPP_CHECK_MATH_SUPPORT

[David Edelsohn]

	Thanks for the GLIBCPP_ENABLE_LONG_DOUBLE test.

	May I create a patch to acinclude.m4 moving the
GLIBCPP_CHECK_MATH_SUPPORT tests after the GLIBCPP_ENABLE_LONG_DOUBLE test
and avoid the basic C math functions long double tests if
GLIBCPP_ENABLE_LONG_DOUBLE already failed?

Thanks, David