Add option to disable rootston build

Compiling the rootston example compositor is useful for testing and
development, but less useful for actual users of wlroots, who might
prefer sway or other compositors.
This commit adds the option --enable-rootston to set weather to include
rootston at compilation or not. The default value is True, so the
current compilation behaviour remains unchanged.

meson.build

@@ -201,7 +201,10 @@ summary = [
 message('\n'.join(summary))
 
 
-subdir('rootston')
+if get_option('enable-rootston')
+	subdir('rootston')
+endif
+
 subdir('examples')
 
 pkgconfig = import('pkgconfig')

meson_options.txt

@@ -4,3 +4,4 @@ option('enable-elogind', type: 'combo', choices: ['auto', 'true', 'false'], valu
 option('enable-xcb_errors', type: 'combo', choices: ['auto', 'true', 'false'], value: 'auto', description: 'Use xcb-errors util library')
 option('enable-xwayland', type: 'boolean', value: true, description: 'Enable support X11 applications')
 option('enable-x11_backend', type: 'boolean', value: true, description: 'Enable X11 backend')
+option('enable-rootston', type: 'boolean', value: true, description: 'Build the rootston example compositor')