build: Add more Meson options for disabling parts of the build

This is all aimed at making it so that sysprof can be built as a
subproject within GLib, with only `libsysprof-capture-4.a` being
produced as output.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

examples/meson.build

@@ -1,3 +1,7 @@
+if get_option('enable_examples')
+
 app = executable('example-app', 'app.c',
   dependencies: [dependency('glib-2.0'), libsysprof_capture_dep],
 )
+
+endif