Update meson.build to use it as a subproject dependency

Extended to also cover the use-case in https://github.com/jpakkane/cppfront

Signed-off-by: KaruroChori <[email protected]>

Author: KaruroChori

meson.build

@@ -28,3 +28,6 @@ install_headers([
     './include/cpp2regex.h2',
     './include/cpp2util.h'
 ])
+
+meson.override_find_program('cppfront', cppfront)
+cpp2_dep = declare_dependency(include_directories: 'include')