Add option to disable examples build

Compiling the examples is useful for testing and development, but less
useful for actual users of wlroots.
This commit adds the option --enable-examples to set weather to include
the examples at compilation or not. The default value is True, so the
current behaviour remains unchanged.

meson.build

@@ -205,7 +205,9 @@ if get_option('enable-rootston')
 	subdir('rootston')
 endif
 
+if get_option('enable-examples')
 	subdir('examples')
+endif
 
 pkgconfig = import('pkgconfig')
 pkgconfig.generate(

meson_options.txt

@@ -5,3 +5,4 @@ option('enable-xcb_errors', type: 'combo', choices: ['auto', 'true', 'false'], v
 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')
+option('enable-examples', type: 'boolean', value: true, description: 'Build example applications')