[Nix-dev] php-packages fix (broken since multiple outputs)

[Marc Weber]

 FYI I have an open PR for this on github as well:
> https://github.com/NixOS/nixpkgs/pull/2464.  Maybe we can compare notes.
This issue is labeled "systemd-fstab-generator doesn't find fsck.ext3"
thus seems unrelated.

There is not much to discuss because
* current situation is broken
* its known why (includes aren't found)
* The best way to fix it is fixing php-config (because it makes all
  packages compile)

=> The only think to discuss is the "assert" stuff so that in future all
packages can be tested by php71Packages or such (better php71.packages)
even better (php7x.packages) -> but that's another story:
  - https://github.com/NixOS/nixpkgs/issues/24432
  - adding tests that the modules actually "work" as expected on multiple
    platforms

The current situation is a stopper for me -> and the fix doesn't make
things worse (testing is as good/bad as it previously was).

Marc Weber