Add a way to force system asio to be used

meson_options.txt

@@ -5,6 +5,18 @@ option(
   description : 'Build a 32-bit host application for hosting 32-bit plugins. See the readme for full instructions on how to use this.'
 )
 
+option(
+  'system-asio',
+  type : 'boolean',
+  value : false,
+  description : '''If set to true, then <asio.hpp> from the standard include
+                   directories will be used in place of a pkgconfig definition,
+                   CMake dependency, or subproject wrap. The asio library does
+                   not come with any build definitions Meson can use to detect
+                   its installed version and location, so this behavior is
+                   behind an option as it's only relevant for distro packaging.'''
+)
+
 option(
   'vst3',
   type : 'boolean',