Raise minimum required CMake version of docopt
for CMake-4 support.
https://bugs.gentoo.org/957536
https://github.com/docopt/docopt.cpp/issues/166

--- a/vendor/docopt/CMakeLists.txt
+++ b/vendor/docopt/CMakeLists.txt
@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.1)
+cmake_minimum_required(VERSION 3.5)
 project(docopt.cpp VERSION 0.6.2)
 
 include(GNUInstallDirs)