Temporary gcc travis-ci build fixes

Recent gcc snapshots (both gcc 7 snapshots and recent gcc 6 stable
branch snapshots) are triggering an upstream gcc bug when -flto is
enabled (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79296).  This has
been hitting the gcc-7 builds for a while now, but is going to start
hitting the debian testing builds in a few days as well.

The issue is triggered by using -flto in combination with structs or
classes declared in a function, as done in test_alias_initialization,
test_isses, test_methods_and_attributes (and possibly more).

I'm subscribed to the upstream bug, and will submit another PR to
reenable LTO once a fixed gcc is available.

The gcc-7 build also generates some warnings; just ignore them for now
(by turning off -Werror).