Clarify build test for presense of 'float copysign(float, float)'.

Clarify build test for presense of 'float copysign(float, float)'.

Clang-cl reports __cplusplus based on the version of clang instead of
reporting the __cplusplus the vc++ version _MSC_VER would.
This is important as the _MSC_VER also indicates which vc++ runtime
library will be used. As a result, do not trust the __cplusplus version
reported by clang-cl.

This change clarifies the intent of
https://skia.googlesource.com/skia/+/c34b0d4e9ad5806c1f882a1f85191f2ea8ddcdba

Committed: https://skia.googlesource.com/skia/+/ef59adba5b5058c60c1b92b17904b2813be644ea

[bungeman-skia, 2014-09-17 16:26:28]

bungeman@google.com changed reviewers:
+ reed@google.com

[bungeman-skia, 2014-09-17 16:26:29]

The idea here is to comment and clarify the intent and reason for
https://codereview.chromium.org/526813002/ .

[reed1, 2014-09-17 17:51:23]

lgtm

[bungeman-skia, 2014-09-17 17:56:34]

The CQ bit was checked by bungeman@google.com

[commit-bot: I haz the power, 2014-09-17 17:56:56]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patchset/576023003/1

[commit-bot: I haz the power, 2014-09-17 18:16:53]

Committed patchset #1 (id:1) as ef59adba5b5058c60c1b92b17904b2813be644ea