Merge #14528: travis: Compile once on xenial

fa4bcaf travis: Compile once on xenial (MarcoFalke)

Pull request description:

  Currently we only build on bionic (since that is also the current gitian environment). However, building on the current and previous Ubuntu LTS should be supported with only system packages and without depends.

Tree-SHA512: bf5725cfb1be09220510d53010c7b7deb20051a9995e39fe5e83505c63db09ac877a41b896c97b253052fefea58ca0a9b6d9c5962a7ac4b258782c476d6ee7c0

.travis.yml

@@ -95,6 +95,15 @@ jobs:
         DEP_OPTS="NO_QT=1 NO_UPNP=1 DEBUG=1 ALLOW_HOST_PACKAGES=1"
         GOAL="install"
         BITCOIN_CONFIG="--enable-zmq --with-gui=qt5 --enable-glibc-back-compat --enable-reduce-exports --enable-debug CXXFLAGS=\"-g0 -O2\""
+# x86_64 Linux (xenial, no depends, only system libs)
+    - stage: test
+      env: >-
+        HOST=x86_64-unknown-linux-gnu
+        DOCKER_NAME_TAG=ubuntu:16.04
+        PACKAGES="python3-zmq qtbase5-dev qttools5-dev-tools libssl-dev libevent-dev bsdmainutils libboost-system-dev libboost-filesystem-dev libboost-chrono-dev libboost-test-dev libboost-thread-dev libdb5.3++-dev libminiupnpc-dev libzmq3-dev libprotobuf-dev protobuf-compiler libqrencode-dev"
+        NO_DEPENDS=1
+        GOAL="install"
+        BITCOIN_CONFIG="--enable-zmq --with-incompatible-bdb --enable-glibc-back-compat --enable-reduce-exports --with-gui=qt5 CPPFLAGS=-DDEBUG_LOCKORDER"
 # x86_64 Linux (no depends, only system libs)
     - stage: test
       env: >-