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

Committed: https://code.google.com/p/v8/source/detail?r=24730

[danno, 2014-10-20 08:46:43]

danno@chromium.org changed reviewers:
+ svenpanne@chromium.org
- danno@chromium.org

[danno, 2014-10-20 08:46:53]

danno@chromium.org changed reviewers:
+ danno@chromium.org

[danno, 2014-10-20 08:46:54]

Sven, could you please take a look?

[Benedikt Meurer, 2014-10-20 11:17:09]

bmeurer@chromium.org changed reviewers:
+ bmeurer@chromium.org

[Benedikt Meurer, 2014-10-20 11:17:10]

lgtm

[Sven Panne, 2014-10-20 11:22:08]

On 2014/10/20 11:17:10, Benedikt Meurer wrote:
> lgtm

Quick explanation: We had a short discussion, and this seems safe because it's a
Solaris-only thing and extending std is undefined behavior, anyway
(http://en.cppreference.com/w/cpp/language/extending_std). It's a bit of a
border case, because it's a C++11 header feature
(http://en.cppreference.com/w/cpp/numeric/math/signbit), but let's see if
anybody complains. :-D

[indutny, 2014-10-20 11:24:21]

On 2014/10/20 11:22:08, Sven Panne wrote:
> On 2014/10/20 11:17:10, Benedikt Meurer wrote:
> > lgtm
> 
> Quick explanation: We had a short discussion, and this seems safe because it's
a
> Solaris-only thing and extending std is undefined behavior, anyway
> (http://en.cppreference.com/w/cpp/language/extending_std). It's a bit of a
> border case, because it's a C++11 header feature
> (http://en.cppreference.com/w/cpp/numeric/math/signbit), but let's see if
> anybody complains. :-D

Yeah, this is what I thought too :) Thank you!

[Benedikt Meurer, 2014-10-20 11:27:47]

Committed patchset #1 (id:1) manually as 24730 (presubmit successful).