travis: Add C++14 build with g++-5

For now, C++14 builds are allowed to fail.

.travis.yml

@@ -9,16 +9,21 @@ compiler:
     - clang
 
 env:
-    - OPT=-O0 NLS=false
-    - OPT=-O2 NLS=false
+    - OPT=-O0 CXXSTD=11 NLS=false
+    - OPT=-O2 CXXSTD=11 NLS=false
+    - OPT=-O2 CXXSTD=1y NLS=false
     - NLS=true
 
 matrix:
   exclude:
     - compiler: gcc
-      env: OPT=-O2 NLS=false
+      env: OPT=-O2 CXXSTD=11 NLS=false
     - compiler: gcc
       env: NLS=true
+    - compiler: clang
+      env: OPT=-O2 CXXSTD=1y NLS=false
+  allow_failures:
+    - env: OPT=-O2 CXXSTD=1y NLS=false
 
 before_install:
     - export TARGETS="wesnoth wesnothd campaignd test"
@@ -26,10 +31,9 @@ before_install:
     - export EXTRA_FLAGS_RELEASE=""
     - export WML_TESTS=true CPP_TESTS=true PLAY_TEST=true MP_TEST=true
     - export WML_TEST_TIME=15
-    - export CXXSTD=11
 
     - if $NLS; then
-          export TARGETS="translations";
+          export TARGETS="translations" CXXSTD=11;
           export WML_TESTS=false CPP_TESTS=false PLAY_TEST=false MP_TEST=false;
       fi
     - if [ "$OPT" == "-O0" ]; then
@@ -40,10 +44,14 @@ before_install:
     - if [[ "$OPT" == "-O0" ]] && [[ "$CXX" == "clang++" ]]; then export EXTRA_FLAGS_RELEASE="-O0 -Wno-deprecated-declarations -Wno-literal-suffix -Wno-deprecated-register"; fi
 
 install:
+    - if [ "$CXXSTD" == "1y" ]; then sudo add-apt-repository ppa:ubuntu-toolchain-r/test -y; fi
     - travis_wait sudo apt-get update -qq
     - travis_wait sudo apt-get install -qq libboost-filesystem-dev libboost-iostreams-dev libboost-random-dev libboost-program-options-dev libboost-regex-dev libboost-system-dev libboost-test-dev libboost-locale-dev libboost-thread-dev libcairo2-dev libfribidi-dev libpango1.0-dev libsdl-image1.2-dev libsdl-mixer1.2-dev libsdl-net1.2-dev libsdl-ttf2.0-dev libsdl2-image-dev libsdl2-mixer-dev libsdl2-net-dev libsdl2-ttf-dev gdb moreutils scons xvfb
 
-    - if [ "$CXX" == "g++" ]; then
+    - if [ "$CXXSTD" == "1y" ]; then
+          sudo apt-get install -qq g++-5;
+          export CXX=g++-5;
+      elif [ "$CXX" == "g++" ]; then
           sudo apt-get install -qq g++-4.7;
           export CXX=g++-4.7;
       fi