api: api_info()['version']

API level is disconnected from NVIM version. The API metadata holds the
current API level, and the lowest backwards-compatible level supported
by this instance.

Release 0.1.6 is the first release that reports the Nvim version and API
level.

    metadata['version'] = {
      major: 0,
      minor: 1,
      patch: 6,
      api_level: 1,
      api_compatible: 0,
      api_prerelease: false,
    }

The API level may remain unchanged across Nvim releases if the API has
not changed.

When changing the API,
    - set NVIM_API_PRERELEASE to true
    - increment NVIM_API_LEVEL (at most once per Nvim version)
    - adjust NVIM_API_LEVEL_COMPAT if backwards-compatibility was broken

api_level_0.mpack was generated from Nvim 0.1.5 with:
    nvim --api-info

1 files changed, 4 insertions, 5 deletions

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 3b139025d7..75c0bd0ecd 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -59,16 +59,15 @@ set_property(CACHE CMAKE_BUILD_TYPE PROPERTY
   STRINGS "Debug" "Dev" "Release" "MinSizeRel" "RelWithDebInfo")
 
 # 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`.
+# version string, else they are combined with the result of `git describe`.
 set(NVIM_VERSION_MAJOR 0)
 set(NVIM_VERSION_MINOR 1)
 set(NVIM_VERSION_PATCH 6)
 set(NVIM_VERSION_PRERELEASE "-dev") # for package maintainers
 
-# Neovim API version. When changing the API, bump CURRENT if
-# PRERELEASE is false, and set PRERELEASE as true
-set(NVIM_API_CURRENT 1)
-set(NVIM_API_COMPATIBILITY 0)
+# API level
+set(NVIM_API_LEVEL 1)         # Bump this after any API change.
+set(NVIM_API_LEVEL_COMPAT 0)  # Adjust this after a _breaking_ API change.
 set(NVIM_API_PRERELEASE true)
 
 file(TO_CMAKE_PATH ${CMAKE_CURRENT_LIST_DIR}/.git FORCED_GIT_DIR)