Add "gamemoderun", a wrapper bash script like "primusrun"

This should remove a point of failure currently where users fail to set LD_PRELOAD correctly, or the path is wrong somehow.

	FWIW: The old method still works, and I've noted that in the README.

data/meson.build

@@ -1,6 +1,7 @@
 data_conf = configuration_data()
 data_conf.set('BINDIR', path_bindir)
 data_conf.set('LIBEXECDIR', path_libexecdir)
+data_conf.set('GAMEMODE_LIB_DIR', path_libdir)
 
 if with_systemd == true
     # Install systemd user unit
@@ -29,5 +30,14 @@ configure_file(
     install_dir: path_polkit_action_dir,
 )
 
+# Install the helper run script
+configure_file(
+    input: 'gamemoderun.in',
+    output: 'gamemoderun',
+    configuration: data_conf,
+    install_dir: 'bin',
+    install_mode: 'rwxr-xr-x',
+)
+
 # Install the man page
 install_man('gamemoded.8')