meson: create dirs and touch /usr

This is the equivalent of $(INSTALL_DIRS) and install-touch-usr-hook.
I did not bother to create the directories into which we install files,
since they will be created anyway.

v2:
- remove bashism

sysctl.d/meson.build

@@ -18,3 +18,6 @@ foreach file : in_files
   install_data(gen,
                install_dir : sysctldir)
 endforeach
+
+meson.add_install_script('sh', '-c',
+                         mkdir_p.format(sysconfdir + '/sysctl.d'))