xcb contains an xml-definition for the GenericEvent extension
but this extension was neither generated nor built.

This patch enables optional building of the GenericEvent extension
with configure option --enable-ge

By default, the GenericEvent extension is not built.
Normally this is not needed by application programs
because there is implicit support for the GE-extension
for the specific events built with this extension.

But it may be useful for X-protocol analyzers and stuff like that.

Signed-off-by: Christian Linhart <chris@demorecorder.com>