Xcode 7 MACOSX_DEPLOYMENT_TARGET and SDK fixes

SDK 10.10 isn't something that exists for Xcode 7, so stop looking for
it and rely on MACOSX_DEPLOYMENT_TARGET instead.

See PR Homebrew/homebrew#50137 Yosemite build failure

Closes Homebrew/homebrew#50355.

Signed-off-by: ilovezfs <ilovezfs@icloud.com>