When building in a git repo:
  - If HEAD corresponds to an annotated tag, (i.e. git_get_exact_tag()
    returns truthy) the current build is considered a "release" build:
    NVIM_VERSION_MEDIUM is directly assigned the tagged version name,
    and NVIM_VERSION_* defines are ignored.
  - If HEAD is not a tagged release, then NVIM_VERSION_MEDIUM is
    directly assigned the result of `git describe`.

If git (or the repo) is not available:
  - The NVIM_VERSION_* defines are used to define NVIM_VERSION_MEDIUM.

Sample outputs for `nvim --version` and `nvim +version`:

    Building with git @ non-tagged commit e66df14:
      NVIM v0.1.0-1-ge66df14 (compiled Nov  1 2015 19:10:30)
      Commit: e66df148f9401be17adab324a6e41d927aae20b3

    Building with git @ v0.1.1 tag:
      NVIM v0.1.1 (compiled Nov  1 2015 19:03:52)
      [no "Commit:" line]

    Building this commit _not_ in a git repo:
      NVIM 0.1.0-dev (compiled Nov  1 2015 19:16:11)
      [no "Commit:" line]

The only thing this affected was the return value of
has('browsefilter').

* Hide commit information from --version if we can't find any (e.g. when
  building from tarball).

To define a release in CMake, set NVIM_VERSION_PRERELEASE to "".
This will modify --version output to:

    * Show annotated Git tag instead of commit hash (NVIM_VERSION_COMMIT).
    * Hide commit date (NVIM_VERSION_BUILD).

RETSIGTYPE, SIGRETURN, and SIGDEFARG are always defined the same, so
inline them for simplicity; SIGDUMMYARG, however, is unused, so remove
it.

Even if these things don't work on windows, the signal handler in
if_cscope.c is already wrapped with '#idef UNIX', as is its only call
site.

Reviewed-by: Justin M. Keyes <justinkz@gmail.com>

HAVE_SYS_TIME_H isn't being used by anything, remove it.

Since gettimeofday() was replaced by os_time in
fb5a786bdb5b7b52b9c36b3eb8b6d2cc002aa8f3 we do not need sys/time.h.