Dealing with Xcode 9 + macOS <= 10.12 and missing functions

[Leonardo Brondani Schenkel]

Hi all,

I have been bitten by this when building Python [1] but I noticed that 
many other ports [2][3][4][5] have been affected by the following:

- You have macOS 10.12 or earlier
- You don't have the command line tools installed
- You update all apps from App Store and end up upgrading Xcode to 9
   (which only includes the 10.13 SDK)
- A port needs to be installed from source
- The port build system is based on autotools or similar
- Autotools tries to check for a function that was introduced in 10.13,
   such as 'utimenstat' or 'futimens'
- The headers claim the function is available, so the source ends up
   using it; it will fail later at either link-time or run-time

[1] https://trac.macports.org/ticket/54893
[2] https://trac.macports.org/ticket/54774
[3] https://trac.macports.org/ticket/54876
[4] https://trac.macports.org/ticket/54875
[5] https://trac.macports.org/ticket/54774

This seems to be an issue in XCode 9 to me, since I believe that when 
setting the deployment target to 10.12 or earlier it should not be 
exposing symbols that are not available at runtime.

Homebrew was affected by this too. Their solution [6] was to set 
environment variables at configure-time to tell autotools not to attempt 
to use the functions. The same can be done in a port-by-port basis on 
MacPorts by using `convigure.env`.

[6] https://github.com/Homebrew/brew/pull/3182

I'm writing this because I am wondering if this should be addressed on a 
port-by-port basis or if it makes sense to have something done globally 
in MacPorts to mitigate the issue?

I have seen some very recent message posted in the list (I don't 
remember if it was in -dev or -users) that touched this subject, but I 
looked at the archives and I could not find it.

Cheers,
// Leonardo.