Fix curl builds for older Xcode platforms
Fix curl builds for older Xcode platforms

Summary: In newer versions of the Darwin kernel, clock_gettime was added but curl doesn't properly configure builds for older versions except when -Werror=partial-availability is defined. This patch allows scraps to run on iOS/tvOS 9 even when compiled with Xcode 8.

Test Plan: _clock_gettime isn't visibile in the output binary

Reviewers: cbrown

Subscribers: #engineering

Differential Revision: https://phabricator.btlive.tv/D1845

bittorrent/scraps,bittorrent/scraps,bittorrent/scraps,bittorrent/scraps