ci: make CI builds fail on compilation warnings

Fio currently builds without warnings using its default options on at
least the platforms where CI is run (Linux, macOS, Windows).
Unfortunately it's easy to overlook changes that start generating
warnings on a platform you don't use regularly. Make this problem more
visible by changing appveyor/travis CI builds to fail when compilation
warnings are produced (but don't make this change for normal manual
builds).

Signed-off-by: Sitsofe Wheeler <sitsofe@yahoo.com>