version: semver.org compliance

We use `git describe` to stamp pre-release versions (dev builds). But
`git describe` uses the result of the most-recent tag (the current
_release_ version)--so we must munge it with the _next_ (i.e.
unreleased) version.

Also fix non-git builds: do not invoke git_describe(NVIM_VERSION_MEDIUM)
if we're not in a git tree, else it gets the dummy value
"HEAD-HASH-NOTFOUND".

Example :version output in non-git build:
  NVIM 0.1.2-dev

Example :version output in git build:
  NVIM v0.1.2-176-g9c3c2b5

1 files changed, 14 insertions, 4 deletions

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 29ef806612..321a919494 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -51,15 +51,25 @@ if(NOT CMAKE_BUILD_TYPE)
   set(CMAKE_BUILD_TYPE "Dev" CACHE STRING "Choose the type of build." FORCE)
 endif()
 
-# If not in a git repo (e.g., a tarball) these tokens set the version string,
-# otherwise the result of `git describe` is used.
+# If not in a git repo (e.g., a tarball) these tokens define the complete
+# version string, else it is combined with the result of `git describe`.
 set(NVIM_VERSION_MAJOR 0)
 set(NVIM_VERSION_MINOR 1)
-set(NVIM_VERSION_PATCH 1)
+set(NVIM_VERSION_PATCH 2)
+set(NVIM_VERSION_PRERELEASE "-dev") # for package maintainers
 
 file(TO_CMAKE_PATH ${CMAKE_CURRENT_LIST_DIR}/.git FORCED_GIT_DIR)
 include(GetGitRevisionDescription)
-git_describe(NVIM_VERSION_MEDIUM)
+get_git_head_revision(GIT_REFSPEC NVIM_VERSION_COMMIT)
+if(NVIM_VERSION_COMMIT) # is a git repo
+  git_describe(NVIM_VERSION_MEDIUM)
+  # `git describe` annotates the most recent tagged release; for pre-release
+  # builds we must replace that with the unreleased version.
+  string(REGEX REPLACE "^v[0-9]+\\.[0-9]+\\.[0-9]+"
+         "v${NVIM_VERSION_MAJOR}.${NVIM_VERSION_MINOR}.${NVIM_VERSION_PATCH}"
+         NVIM_VERSION_MEDIUM
+         ${NVIM_VERSION_MEDIUM})
+endif()
 
 set(NVIM_VERSION_BUILD_TYPE "${CMAKE_BUILD_TYPE}")
 # NVIM_VERSION_CFLAGS set further below.