[v3] special math vs. C++0x test

[Benjamin Kosnik]

> I'm confused.  Did the TR1 special math functions just make it into the 
> standard?
> 
> I haven't seen anything later than 03-2007 on open-std.org.

This is confusing: a negative test that is xfailed. Translated, it means 
that it's supposed to not compile, but does.

This is similar to the negative tests here:

	* testsuite/20_util/headers/type_traits/types_std_c++0x_neg.cc:
	Add a temporary xfail to this test for all platforms.

Does this make it clear?

best,
benjamin