build(vendor): vendor libraries now install in a dedicated directory

We now install vendor libraries into includedir/fourdst/vendor to avoid conflicts with user installed libraries. Note that currently this only applies to headers and not compiled shared libraries

BREAKING CHANGE:

build-config/CLI11/meson.build

@@ -1,2 +1,8 @@
 cli11_proj = subproject('cli11')
-cli11_dep = cli11_proj.get_variable('CLI11_dep')
+cli11_dep = cli11_proj.get_variable('CLI11_dep')
+
+install_subdir(
+    meson.project_source_root() / 'subprojects' / 'CLI11-2.6.1' / 'include',
+    install_dir: get_option('includedir') / 'fourdst' / 'vendor',
+    strip_directory: true
+)