build: Support the meson build system

Meson is on track to replace autotools as the build system of choice,
so support it in addition to autotools. If all goes well, we'll
eventually be able to drop the latter ...

https://bugzilla.gnome.org/show_bug.cgi?id=783229

docs/reference/st/st-docs.sgml

@@ -47,10 +47,15 @@
       <xi:include href="xml/st-texture-cache.xml"/>
     </chapter>
   </part>
+  <!-- FIXME:
+       Not generated at the moment, find out whether to blame gtk-doc or meson
+  -->
+  <!--
   <chapter id="object-tree">
     <title>Object Hierarchy</title>
      <xi:include href="xml/tree_index.sgml"/>
   </chapter>
+  -->
   <index id="api-index-full">
     <title>API Index</title>
     <xi:include href="xml/api-index-full.xml"><xi:fallback /></xi:include>