Add pty to Glibc modulemap

[aciidgh]

What's in this pull request?

This adds the header pty.h to the Glibc modulemap.

Resolved bug number: (SR-)

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[ddunbar]

It's unfortunate we can't make this a directive in the submodule, as this is going to cause everything to link with util. However, that is consistent with the current Glibc module design (and Darwin), so it does seem to fit into the current model.

[aciidgh]

Do you know why it doesn't link at all if its in the submodule?

[ddunbar]

It is a limitation of the current Clang modules implementation, unfortunately. My understanding it is non-trivial to fix that.

[ddunbar]

This looks reasonable to me given the information in https://www.gnu.org/software/gnulib/manual/html_node/openpty.html, the imposition of -lutil on every client of Glibc is unfortunate but as noted inline I'm not sure we have a design for an alternative (other than simply not supporting this function).

@gribozavr can you review or delegate to someone else?

[ddunbar]

@swift-ci please smoke test

[aciidgh]

@gribozavr friendly ping, swiftpm has two PR dependant on this 😄

[gribozavr]

@aciidb0mb3r Could you also add includes for utmp.h, which defines login() and logout() that are included in -lutil?

Could you also add a fixme near the link "util" line that explains that this dependency is only needed for a few rarely-used functions and is not usually necessary?

[aciidgh]

@gribozavr Done

[gribozavr]

@swift-ci Please smoke test

[aciidgh]

is the Linux failure is related to this?

[gribozavr]

Does not look related indeed. Merging!

If you want to submit swiftpm fixes that depend on this change, I would recommend submitting a PR of this change for the swift-3.0-branch, this way we would be able to cherry-pick dependent swiftpm changes, if such a need arises.

[aciidgh]

@gribozavr Thanks!
I opened a PR on swift 3 branch #4039