Summary:
In ffc23270, an attempt was made at
adding a Haskell wrapper around the C `timer_t` type. Unfortunately, GHC's
autoconf macros weren't sophisticated enough at the time to properly detect
that `timer_t` is represented by a `void *` (i.e., a pointer) on most OSes.

This is a second attempt at `CTimer`, this time using `AC_COMPILE_IFELSE` to
detect if a type is a pointer type by compiling the following program:

```
type val;
*val;
```

This also only derives a small subset of class instances for `CTimer` that are
known to be compatible with `Ptr` using a new `OPAQUE_TYPE_WITH_CTYPE` macro.

Test Plan: ./validate

Reviewers: erikd, hvr, austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2952

GHC Trac Issues: #12795, #12998