platform: fix build on SmartOS

platform: fix build on SmartOS

With -std=g++0x, `signbit` is no longer a define, and is an actual
function declared in C++-specific headers. Thus checking it's presence
with ifdef will no longer work.

Considering that g++0x implies newer compiler, there should not be the
case where it won't be present anymore.

BUG=
R=bmeurer@chromium.org, danno

Review URL: https://codereview.chromium.org/642203003

Patch from Fedor Indutny <fedor.indutny@gmail.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24730 ce2b1a6d-e550-0410-aec6-3dcde31c8c00