build: run git-describe for dev version during build (#11117)

This avoids invoking CMake after a new commit, which might take 15s on
some systems.

Skipped on CMake < 3.2.0 (missing BYPRODUCTS support).

Co-Authored-By: Justin M. Keyes <justinkz@gmail.com>

scripts/update_version_stamp.lua

@@ -0,0 +1,45 @@
+#!/usr/bin/env lua
+--
+-- Script to update the Git version stamp during build.
+-- This is called via the custom update_version_stamp target in
+-- src/nvim/CMakeLists.txt.
+--
+-- arg[1]: file containing the last git-describe output
+-- arg[2]: file in which to update the version string
+
+local function die(msg)
+  print(string.format('%s: %s', arg[0], msg))
+  -- No error, fall back to using generated "-dev" version.
+  os.exit(0)
+end
+
+if #arg ~= 2 then
+  die(string.format("Expected two args, got %d", #arg))
+end
+
+local stampfile = arg[1]
+local stamp = io.open(stampfile, 'r')
+if stamp then
+  stamp = stamp:read('*l')
+end
+
+local current = io.popen('git describe --dirty'):read('*l')
+if not current then
+  die('git-describe failed')
+end
+
+if stamp ~= current then
+  if stamp then
+    print(string.format('git version changed: %s -> %s', stamp, current))
+  end
+  local new_lines = {}
+  local versiondeffile = arg[2]
+  for line in io.lines(versiondeffile) do
+    if line:match("NVIM_VERSION_MEDIUM") then
+      line = '#define NVIM_VERSION_MEDIUM "'..current..'"'
+    end
+    new_lines[#new_lines + 1] = line
+  end
+  io.open(versiondeffile, 'w'):write(table.concat(new_lines, '\n') .. '\n')
+  io.open(stampfile, 'w'):write(current)
+end