Don't show links to extension functions in non-proportional font.

Doxyfile

@@ -201,7 +201,7 @@ ALIASES                = \
     "collisionoperator{2}=@relates \1\n@brief Collision of %\1 and %\2\n@see \2::operator%(const \1&) const" \
     "todoc=@xrefitem todoc \"Documentation todo\" \"Documentation-related todo list\"" \
     "fn_gl{1}=<a href=\"http://www.opengl.org/sdk/docs/man4/xhtml/gl\1.xml\">gl\1()</a>" \
-    "fn_gl_extension{3}=<a href=\"http://www.opengl.org/registry/specs/\2/\3.txt\"><tt>gl\1<b></b>\2()</tt></a>" \
+    "fn_gl_extension{3}=<a href=\"http://www.opengl.org/registry/specs/\2/\3.txt\">gl\1<b></b>\2()</a>" \
     "def_gl{1}=`GL_\1`" \
     "requires_gl=@xrefitem requires-gl \"Requires desktop OpenGL\" \"Functionality requiring desktop OpenGL (not available on OpenGL ES)\" Not available on OpenGL ES." \
     "requires_gl30=@xrefitem requires-gl30 \"Requires OpenGL 3.0\" \"Functionality requiring OpenGL 3.0\"" \