Implement directory monitoring using septag/dmon

Use a notification based directory monitoring based on the
septag/dmon lirbary instead of periodically rescan the whole
project's tree.

meson.build

@@ -45,6 +45,7 @@ endif
 if not get_option('source-only')
     libm = cc.find_library('m', required : false)
     libdl = cc.find_library('dl', required : false)
+    threads_dep = dependency('threads')
     lua_dep = dependency('lua5.2', fallback: ['lua', 'lua_dep'],
         default_options: ['shared=false', 'use_readline=false', 'app=false']
     )
@@ -57,7 +58,7 @@ if not get_option('source-only')
         ]
     )
 
-    lite_deps = [lua_dep, sdl_dep, reproc_dep, pcre2_dep, libm, libdl]
+    lite_deps = [lua_dep, sdl_dep, reproc_dep, pcre2_dep, libm, libdl, threads_dep]
 
     if host_machine.system() == 'windows'
         # Note that we need to explicitly add the windows socket DLL because