configure.ac: unredundantize string comparisons

[ "x$foo" = "xbar" ] is silly. To be empty-string-safe, you need to
either put quotes around $foo, or concatenate something to it, but
there's no reason to do both.
tests: Use the TAP driver
gnutls: Prevent use of a destroyed GSource in a callback

It’s possible for the GTlsConnectionGnutlsSource to be destroyed in its
user-provided callback function. If gnutls_source_sync() is called after
this, it ends up attempting to modify the GSource after it’s been
destroyed, which is disallowed.

Bail out of gnutls_source_sync() early if the source has already been
destroyed.

https://bugzilla.gnome.org/show_bug.cgi?id=723774
fix distcheck
2.39.90