Travis: test more g++ and clang versions

Also use the build compiler, not the system default, for tests. This was already done
with gcc (by placing a symlink named 'gcc' pointing to gcc-5 in a directory ahead of /usr/bin
in the path), but g++ tests were using the image default compiler -- g++-4.8 for Ubuntu Trusty.

Author: smowton

.travis.yml

@@ -103,7 +103,9 @@ jobs:
             - parallel
             - libc6-dev-i386
       before_install:
-        - mkdir bin ; ln -s /usr/bin/gcc-5 bin/gcc
+        - mkdir bin
+        - ln -s /usr/bin/gcc-5 bin/gcc
+        - ln -s /usr/bin/g++-5 bin/g++
       # env: COMPILER=g++-5 SAN_FLAGS="-fsanitize=undefined -fno-sanitize-recover -fno-omit-frame-pointer"
       env:
         - COMPILER="ccache /usr/bin/g++-5"
@@ -136,7 +138,9 @@ jobs:
             - libubsan0
             - libc6-dev-i386
       before_install:
-        - mkdir bin ; ln -s /usr/bin/gcc-5 bin/gcc
+        - mkdir bin
+        - ln -s /usr/bin/gcc-5 bin/gcc
+        - ln -s /usr/bin/g++-5 bin/g++
       # env: COMPILER=g++-5 SAN_FLAGS="-fsanitize=undefined -fno-sanitize-recover -fno-omit-frame-pointer"
       env:
         - COMPILER="ccache /usr/bin/g++-5"
@@ -157,12 +161,15 @@ jobs:
           packages:
             - libwww-perl
             - clang-3.7
+            - g++-5
             - libstdc++-5-dev
             - libubsan0
             - parallel
             - libc6-dev-i386
       before_install:
-        - mkdir bin ; ln -s /usr/bin/gcc-5 bin/gcc
+        - mkdir bin
+        - ln -s /usr/bin/gcc-5 bin/gcc
+        - ln -s /usr/bin/c++-5 bin/g++
         - export CCACHE_CPP2=yes
       # env: COMPILER=clang++-3.7 SAN_FLAGS="-fsanitize=undefined -fno-sanitize-recover=undefined,integer -fno-omit-frame-pointer"
       env:
@@ -184,11 +191,14 @@ jobs:
           packages:
             - libwww-perl
             - clang-3.7
+            - g++-5
             - libstdc++-5-dev
             - libubsan0
             - libc6-dev-i386
       before_install:
-        - mkdir bin ; ln -s /usr/bin/gcc-5 bin/gcc
+        - mkdir bin
+        - ln -s /usr/bin/gcc-5 bin/gcc
+        - ln -s /usr/bin/g++-5 bin/g++
         - export CCACHE_CPP2=yes
       # env: COMPILER=clang++-3.7 SAN_FLAGS="-fsanitize=undefined -fno-sanitize-recover=undefined,integer -fno-omit-frame-pointer"
       env:
@@ -212,14 +222,74 @@ jobs:
             - g++-5
             - libc6-dev-i386
       before_install:
-        - mkdir bin ; ln -s /usr/bin/gcc-5 bin/gcc
+        - mkdir bin
+        - ln -s /usr/bin/gcc-5 bin/gcc
+        - ln -s /usr/bin/g++-5 bin/g++
       install:
         - ccache -z
         - ccache --max-size=1G
         - cmake -H. -Bbuild '-DCMAKE_BUILD_TYPE=Release'  '-DCMAKE_CXX_COMPILER=/usr/bin/g++-5'
         - cmake --build build -- -j4
       script: (cd build; ctest -V -L CORE -j2)
 
+    # cmake build using g++-7
+    - stage: Test different OS/CXX/Flags
+      os: linux
+      compiler: gcc
+      cache: ccache
+      env:
+        - BUILD_SYSTEM=cmake
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-7
+            - libc6-dev-i386
+      before_install:
+        - mkdir bin
+        - ln -s /usr/bin/gcc-7 bin/gcc
+        - ln -s /usr/bin/g++-7 bin/g++
+      install:
+        - ccache -z
+        - ccache --max-size=1G
+        - cmake -H. -Bbuild '-DCMAKE_BUILD_TYPE=Release'  '-DCMAKE_CXX_COMPILER=/usr/bin/g++-7'
+        - cmake --build build -- -j4
+      script: (cd build; ctest -V -L CORE -j2)
+
+    # cmake build using clang++-6
+    - stage: Test different OS/CXX/Flags
+      os: linux
+      compiler: clang
+      cache: ccache
+      env:
+        - BUILD_SYSTEM=cmake
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+            - llvm-toolchain-trusty-6.0
+          packages:
+            - libwww-perl
+            - g++-5
+            - clang-6.0
+            - libstdc++-5-dev
+            - libubsan0
+            - parallel
+            - libc6-dev-i386
+      before_install:
+        - mkdir bin
+        # Use gcc/g++ 5 for tests, as Clang doesn't work yet
+        - ln -s /usr/bin/gcc-5 bin/gcc-5
+        - ln -s /usr/bin/g++-5 bin/g++-5
+      install:
+        - ccache -z
+        - ccache --max-size=1G
+        - cmake -H. -Bbuild '-DCMAKE_BUILD_TYPE=Release'  '-DCMAKE_CXX_COMPILER=/usr/bin/clang++-6.0'
+        - cmake --build build -- -j4
+      script: (cd build; ctest -V -L CORE -j2)
+
+    # cmake build on OSX, using default clang
     - stage: Test different OS/CXX/Flags
       os: osx
       compiler: clang