[C++] Upgrade thirdparty gtest to 1.8.0

[asfimport]

This also resolves some linker issues I've run into on OS X with Xcode 6.4

Reporter: Wes McKinney / @wesm
Assignee: Wes McKinney / @wesm

Related issues:

• Arrow 0.3 release (blocks)

_{Note: This issue was originally created as ARROW-832. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
PR: #549

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 549
#549