meson: parse lua version when the binary is unversioned.

I had the case on my machine where `lua` was version 5.4.4 and the
lua-lgi test succeeded fine, but then the demo plug-in would fail to run
with the error message from #11876.

Let's parse the output of `lua -v` to detect the version ourselves and
therefore be able to output proper error messages for packagers not to
assume that the Lua interpreter they are using will work fine.

meson.build

@@ -1126,12 +1126,35 @@ if get_option('lua').allowed()
     lua = find_program(lua_bin, required: false)
 
     if lua.found() and meson.can_run_host_binaries()
+      if lua_bin == 'lua'
+        is_supported_lua = false
+        lua_cmd = run_command(lua, '-v', check: false)
+        if lua_cmd.returncode() == 0
+          message('Found Lua: @0@'.format(lua_cmd.stdout().strip()))
+          lua_stdout = lua_cmd.stdout().split()
+          if lua_stdout[0].to_lower() == lua_bin
+            message('Parsed Lua version: @0@'.format(lua_stdout[1]))
+            # We only want any lua version 5.1.x
+            if lua_stdout[1].version_compare('>=5.1.0') and lua_stdout[1].version_compare('<5.2.0')
+              is_supported_lua = true
+            else
+              warning('Unsupported Lua version (!= 5.1): @0@'.format(lua_stdout[1]))
+            endif
+          endif
+        else
+          warning('Failed to run `lua -v`')
+        endif
+      else
+        is_supported_lua = true
+      endif
+
+      if is_supported_lua
         have_lua_lgi = run_command(lua, '-e',
                                    '''
                                    local lgi = require 'lgi'
                                    ''',
-      check: false
-      ).returncode() == 0
+                                   check: false).returncode() == 0
+      endif
 
       if have_lua_lgi
         break
@@ -1142,8 +1165,8 @@ if get_option('lua').allowed()
   have_lua = get_option('lua').enabled() or have_lua_lgi
   if have_lua and not have_lua_lgi
     lua_warning = '''
-      Neither Luajit nor Lua, with lua-lgi support, was found. Or you are
-      cross-compiling so we couldn't test lua-lgi support.
+      Neither Luajit nor Lua 5.1, with lua-lgi support, was found.
+      Or you are cross-compiling so we couldn't test lua-lgi support.
       Lua plug-ins will be installed anyway but you should make sure that
       luajit or lua and that LGI are available at installation, otherwise
       installed plug-ins won't be usable.