build: Deprecate -Druntime_libdir option

It’s been broken since we ported to Meson and nobody has complained, so
let’s deprecate it this cycle and remove it in GLib ≥ 2.78.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Fixes: #2786

docs/reference/glib/building.xml

@@ -334,23 +334,6 @@
         <filename>meson-logs</filename> directory.
       </para>
     </formalpara>
-
-    <formalpara>
-      <title><option>-Druntime_libdir=RELPATH</option></title>
-
-      <para>
-        Allows specifying a relative path to where to install the runtime
-        libraries (meaning library files used for running, not developing,
-        GLib applications). This can be used in operating system setups where
-        programs using GLib needs to run before e.g. <filename>/usr</filename>
-        is mounted.
-        For example, if <varname>LIBDIR</varname> is <filename>/usr/lib</filename> and
-        <filename>../../lib</filename> is passed to
-        <option>-Druntime_libdir</option> then the
-        runtime libraries are installed into <filename>/lib</filename> rather
-        than <filename>/usr/lib</filename>.
-      </para>
-    </formalpara>
   </refsect1>
 
 </refentry>

meson_options.txt

@@ -1,7 +1,8 @@
 option('runtime_libdir',
        type : 'string',
        value : '',
-       description : 'install runtime libraries relative to libdir')
+       description : 'install runtime libraries relative to libdir',
+       deprecated: true)
 
 option('iconv',
        type : 'combo',