reject macros in math decl check

The C++ headers #undef the functions we are testing for, just in case
they're implemented as macros, so do that in the cross math decl tests
as well.


for  libstdc++-v3/ChangeLog

	* crossconfig.m4 (GLIBCXX_CHECK_MATH_DECL): Reject macros.
	* configure: Rebuild.