meson: Fix compatibility error with CMake >=4.0

Fixes #371.

Author: arch1t3cht

meson.build

@@ -146,6 +146,8 @@ else
     opt_var = cmake.subproject_options()
     opt_var.set_override_option('cpp_std', 'c++20')
     opt_var.add_cmake_defines({
+        'CMAKE_POLICY_VERSION_MINIMUM': '3.31',
+
         'wxBUILD_INSTALL': false,
         'wxBUILD_PRECOMP': 'OFF',       # otherwise breaks project generation w/ meson
         'wxBUILD_SHARED': build_shared,