simple config speedup GLIBCPP_CHECK_MATH_SUPPORT

[Gabriel Dos Reis]

Benjamin Kosnik <bkoz@redhat.com> writes:

| 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.

I'm all for that!

-- Gaby
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