Build System: Fix --disable-xmldoc option.

The configure option to disable XML documentation does not currently
work.  This patch makes it effective, but also causes an ABI change by
removing the ast_xmldoc_* symbols.  Disabling xmldoc also prevents docs
from being automatically generated, but they can still be manually
generated with 'make doc/core-en_US.xml'.

ASTERISK-26639

Change-Id: Ifac562340c09f80c83e0203de098fcac93bf8c44