Travis CI folow up

Fix ENV_VARS passing to `docker` so `CC` is set to `clang`.
Fix asan compiler option.
Specify all compiler options in `CFLAGS`.

Related discussion https://github.com/libbpf/libbpf/pull/36#discussion_r276522335

travis-ci/managers/xenial.sh

@@ -10,6 +10,8 @@ source "$(dirname $0)/travis_wait.bash"
 
 cd $REPO_ROOT
 
+CFLAGS="-g -O2 -Werror -Wall -fsanitize=address,undefined"
 mkdir build
-make CFLAGS='-Werror -Db_sanitize=address,undefined' -C ./src -B OBJDIR=../build
+cc --version
+make CFLAGS="${CFLAGS}" -C ./src -B OBJDIR=../build
 rm -rf build