travis: Compile once on trusty

.travis.yml

@@ -99,6 +99,17 @@ jobs:
         GOAL="install"
         BITCOIN_CONFIG="--enable-zmq --with-gui=qt5 --enable-glibc-back-compat --enable-reduce-exports --enable-debug CXXFLAGS=\"-g0 -O2\""
 
+    - stage: test
+      name: 'x86_64 Linux  [GOAL: install]  [trusty]  [no depends, only system libs]'
+      env: >-
+        HOST=x86_64-unknown-linux-gnu
+        DOCKER_NAME_TAG=ubuntu:14.04
+        PACKAGES="python3-zmq qtbase5-dev qttools5-dev-tools libicu-dev libpng-dev libssl-dev libevent-dev bsdmainutils libboost-system-dev libboost-filesystem-dev libboost-chrono-dev libboost-test-dev libboost-thread-dev libdb5.1++-dev libminiupnpc-dev libzmq3-dev libprotobuf-dev protobuf-compiler libqrencode-dev"
+        NO_DEPENDS=1
+        RUN_FUNCTIONAL_TESTS=false
+        GOAL="install"
+        BITCOIN_CONFIG="--enable-zmq --with-incompatible-bdb --with-gui=no"
+
     - stage: test
       name: 'x86_64 Linux  [GOAL: install]  [xenial]  [no depends, only system libs, sanitizers: thread (TSan), no wallet]'
       env: >-