travis: enable more warnings, add -Werror

This will ensure that our code stays free of warnings, at least for the
warnings that we have enabled. (Using _FORTIFY_SOURCE > 0 leads to more
warnings, some of them are harder to address.)

.travis.yml

@@ -42,6 +42,6 @@ install:
   - sudo /bin/sh -c 'cpanm -n -v X11::XCB || true'
   - sudo /bin/sh -c 'cpanm -n -v AnyEvent::I3 || true'
 script:
-  - make -j
+  - CFLAGS="-Wformat -Wformat-security -Wextra -Wno-unused-parameter -Werror" CPPFLAGS="-D_FORTIFY_SOURCE=0" make -j
   - (cd testcases && xvfb-run ./complete-run.pl --parallel=1 || (cat latest/complete-run.log; false))
   - clang-format-3.5 -i $(find . -name "*.[ch]" | tr '\n' ' ') && git diff --exit-code || (echo 'Code was not formatted using clang-format!'; false)