The Trusty image used by Travis was updated in December and now has Clang
5.0.0 installed. So this workaround is not necessary anymore.
This reverts commit f4bd46764143744202b817cf7268aa9e6f4ab5f7.
This is a follow up on the issue documented in the previous commit.
To build with -Werror and Clang 3.9 we'd have to change all enum arguments
that are used as last argument before ... to e.g. u_int, which affects
quite a lot of places (crypto-factory, MODP_CUSTOM constructors, auth-cfg,
bus, vici-builder, vici-message). Besides that it doesn't look as nice
it also seems a bit too much hassle just to cater to the whims of a
particular version of one compiler, so we just don't build with that
version on Travis and use 4.0 instead.
Since the move to Google Compute Engine (GCE) IPv6 has been disabled
on the build hosts, causing several tests to fail. Lets try to get at
least local IPv6 connectivity up again.
But disable the gcrypt plugin, as it causes leaks.
Also disable the backtraces by libunwind as they seem to cause
threads to get cleaned up after the leak detective already has been
disabled, which leads to invalid free()s.
