Instead of relying on a separate file that requires being update every
time we add a new public function we should use compiler annotations to
let the linker know which symbols are public and exported.

In order to achieve this we have to:

 * check for the visibility=hidden attribute
 * add -fvisibility=hidden to the linker flags
 * add a macro to annotate all public symbols

While we're at it, we should copy the versioned symbols macro layout
already used by GLib, GTK+, and other G* libraries, including the
ability to express the range of allowed versions of JSON-GLib that
third party code can compile against.