build: Use `git describe` for version strings.

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]

1 files changed, 13 insertions, 11 deletions

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 4e43bbcd41..f11b9995c0 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -47,24 +47,26 @@ if(NOT CMAKE_BUILD_TYPE)
 endif()
 
 # Version tokens
+#   - In a git repo, these tokens are _ignored_.
+#     - If the current HEAD is tagged, the tag name is used.
+#     - Otherwise the result of `git describe` is used.
+#   - If not in a git repo (e.g. a tarball) these tokens set the version string.
 set(NVIM_VERSION_MAJOR 0)
 set(NVIM_VERSION_MINOR 1)
 set(NVIM_VERSION_PATCH 0)
+set(NVIM_VERSION_PRERELEASE "-dev")
 
 file(TO_CMAKE_PATH ${CMAKE_CURRENT_LIST_DIR}/.git FORCED_GIT_DIR)
 include(GetGitRevisionDescription)
-if(NVIM_VERSION_PRERELEASE)
-  get_git_head_revision(GIT_REFSPEC NVIM_VERSION_COMMIT)
-
-  # TODO(justinmk): UTC time would be nice here #1071
-  git_timestamp(GIT_TIMESTAMP)
-  if(GIT_TIMESTAMP)
-    set(NVIM_VERSION_BUILD "+${GIT_TIMESTAMP}")
+get_git_head_revision(GIT_REFSPEC NVIM_VERSION_COMMIT)
+if(NVIM_VERSION_COMMIT)    # is a git repo
+  git_get_exact_tag(NVIM_VERSION_MEDIUM)
+  if(NVIM_VERSION_MEDIUM)  # is a tagged release
+    unset(NVIM_VERSION_COMMIT)
+  else()                   # is a dev build
+    git_describe(NVIM_VERSION_MEDIUM)
+    get_git_head_revision(GIT_REFSPEC NVIM_VERSION_COMMIT)
   endif()
-else()
-  # If possible, get the Git tag for the current revision.
-  git_get_exact_tag(NVIM_VERSION_COMMIT)
-  set(NVIM_VERSION_BUILD "")
 endif()
 
 set(NVIM_VERSION_BUILD_TYPE "${CMAKE_BUILD_TYPE}")