gmmproc: New and improved _CONV_*_ENUM m4 macros

* tools/m4/convert_base.m4: In _CONV_ENUM and _CONV_INCLASS_ENUM, add an
optional argument, making it possible to define useful conversions
when the C name of the enum is not the concatenation of C++ namespace +
enum_name or namespace + class_name + enum_name.
* tools/m4/convert_gio.m4: Add _CONV_GIO_ENUM, _CONV_GIO_DBUS_ENUM,
_CONV_GIO_INCLASS_ENUM and _CONV_GIO_DBUS_INCLASS_ENUM.
* tools/m4/convert_glib.m4: In _CONV_GLIB_ENUM and _CONV_GLIB_INCLASS_ENUM,
add an optional argument like in convert_base.m4.