Tamar Christina authored Feb 23, 2017 and Ben Gamari committed Feb 26, 2017

We have a unfortunate workaround in place for the fact that most
packages out there use POSIX names for symbols even on Windows.  This
means that we have to recognize e.g. both `_ungetch` and `ungetch`.

The former is the actual symbol name on windows and the latter is the
POSIX variant. The problem is that on normal windows programs `ungetch`
should not be in the global namespace.

To work around this, we now mark the deprecated symbols as weak symbols
in the global namespace. This provides the flexibility we need:

* If you use the symbol without defining it, we assume you meant to use
  the POSIX variant.
* If you actually define the symbol, we'll hence forth use that
  definition and assume you didn't mean to use POSIX code. This is how
  MingW64's wrapper also works.

This requires D3028.

Fixes #13210.

Test Plan: ./validate

Reviewers: austin, bgamari, erikd, simonmar

Reviewed By: bgamari

Subscribers: thomie, #ghc_windows_task_force

Differential Revision: https://phabricator.haskell.org/D3154