Audio: added OpenAL Doxygen page.

Now the \requires_al_extension command works.

Doxyfile

@@ -280,6 +280,7 @@ ALIASES                = \
     "fn_alc{1}=`alc\1()`" \
     "def_al{1}=`AL_\1`" \
     "def_alc{1}=`ALC_\1`" \
+    "requires_al_extension=@xrefitem requires-al-extension \"Requires OpenAL extension\" \"Functionality requiring specific OpenAL extension\"" \
     "al_extension{2}=<a href=\"http://icculus.org/alextreg/index.php?operation=op_showext&extname=AL_\1_\2\"><tt>AL_\1_\2</tt></a>" \
     "alc_extension{2}=<a href=\"http://icculus.org/alextreg/index.php?operation=op_showext&extname=ALC_\1_\2\"><tt>ALC_\1_\2</tt></a>"
 

doc/openal.dox

@@ -0,0 +1,50 @@
+/*
+    This file is part of Magnum.
+
+    Copyright © 2010, 2011, 2012, 2013, 2014, 2015
+              Vladimír Vondruš <mosra@centrum.cz>
+
+    Permission is hereby granted, free of charge, to any person obtaining a
+    copy of this software and associated documentation files (the "Software"),
+    to deal in the Software without restriction, including without limitation
+    the rights to use, copy, modify, merge, publish, distribute, sublicense,
+    and/or sell copies of the Software, and to permit persons to whom the
+    Software is furnished to do so, subject to the following conditions:
+
+    The above copyright notice and this permission notice shall be included
+    in all copies or substantial portions of the Software.
+
+    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
+    THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+    FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+    DEALINGS IN THE SOFTWARE.
+*/
+
+namespace Magnum {
+
+/** @page openal OpenAL
+@brief State of OpenAL support, version and extension requirements.
+
+@todo OpenAL mapping table
+
+State of implementation of particular OpenAL extensions is in the following
+table.
+
+-   @subpage openal-support
+
+@section openal-required-extensions Extension requirements
+
+The @ref Audio library requires at least stock OpenAL 1.1 with no extensions,
+but some specific functionality has greater requirements. Following is the list
+of features requiring specific OpenAL extensions.
+
+-   @subpage requires-al-extension
+
+@page requires-al-extension Functionality requiring specific OpenAL extension
+
+*/
+
+}