travis: compile with -O1 with clang

travis: compile with -O1 with clang

libc in the Debian container got updated to 2.31, which in combination
with clang-9 triggers systemd/systemd#14865.

This has been fixed by https://reviews.llvm.org/D74712 which is (to my
knowledge) included in clang-10. To mitigate this without upgrading to
clang-10 we can compile with -O1 which works around it as well, see
https://bugzilla.redhat.com/show_bug.cgi?id=1803203.