ssl_gnutls: Fix some uninitialized memory warnings
Makefile: Use wildcard for headers to avoid missing files

The following headers were missing:

dcc.h otr.h lib/base64.h lib/oauth2.h lib/oauth.h lib/xmltree.h
Prevent sone "Source ID ## was not found..." warnings

These appear in glib 2.40, and, well, are pretty much useless. But
people complain about them anyway.

Probably fixes trac ticket 1151, at least partially.