Do not expect a copysign function to be defined in <cmath> with clang-cl

clang-cl defines __cplusplus to 201103L, but it uses the runtime library
provided by MSVC, so the copysign function will not be available there.

BUG=skia:
R=reed@google.com

Author: ehsan.akhgari@gmail.com

Review URL: https://codereview.chromium.org/526813002
1 file changed