Fix cmake check for futimens when deploying to earlier macOS releases.

macOS 10.13 added a new API (futimens). This API is only available on macOS 10.13
and later, but the cmake check we have in place only tests if the symbol is
present and ignores the availability attribute. Luckily we have new warning for
this and by making this warning an error the cmake check will return the correct
result.

See also rdar://problem/33992750.

Differential Revision: https://reviews.llvm.org/D37027

llvm-svn: 311949