build(install): vendored install directories

all vendor libraries are all now installed in a contained vendor folder and in a way which is compatible with libconfig being used as a subproject

build-config/tomlpp/meson.build

@@ -1,7 +1,9 @@
-tomlpp_dep = dependency('tomlplusplus', required: true, static: true).as_system()
+tomlpp_inc_dir = include_directories('vendor/include')
 
 install_subdir(
-    meson.project_source_root() / 'subprojects' / 'tomlplusplus-3.4.0' / 'include',
+    'vendor/include',
     install_dir: get_option('includedir') / 'fourdst' / 'vendor',
     strip_directory: true
-)
+)
+
+