Introduce Travis CI tests

In order to libbpf to be used in systemd some testing is required, see related
discussions in https://github.com/systemd/systemd/pull/12151 and
https://github.com/libbpf/libbpf/pull/29
The tests introduced here mirrors the tests of systemd:
For Debian: build with gcc, gcc + asan, clang, clang + asan
Debian tests use `docker` virtualization
Fror Ubuntu Xenial: build with gcc

The differences:
Install only libelf and it's dependencies.
Instead of Meson build system `make` is used, so `make` remains the preferred
method of building and `meson.build` doesn't get rooted in `libbpf`.

`travis_wait.bash` is kept as a workaround for
https://github.com/travis-ci/travis-ci/issues/9979

An example of testing UI: https://travis-ci.org/wat-ze-hex/libbpf

travis-ci/managers/xenial.sh

@@ -0,0 +1,15 @@
+#!/bin/bash
+set -e
+set -x
+
+apt-get update
+apt-get -y build-dep libelf-dev
+apt-get install -y libelf-dev
+
+source "$(dirname $0)/travis_wait.bash"
+
+cd $REPO_ROOT
+
+mkdir build
+make CFLAGS='-Werror -Db_sanitize=address,undefined' -C ./src -B OBJDIR=../build
+rm -rf build