simple config speedup GLIBCPP_CHECK_MATH_SUPPORT

[Benjamin Kosnik]

Hey people. 

I'm now in the camp of grouping the math checks into two segments, the

sin camp

and the

_sin camp.

This change alone will probably speed up the libstdc++-v3 configure by
minutes (feels like years) on slow machines. If the first is found,
the second group shouldn't be checked.

Any objections?

Any volunteers?

If not I'll try to do this tonight.

-benjamin