add more compilers to travis

Squashed commit of the following:

commit 91dda92
Author: Fytch <[email protected]>
Date:   Wed Dec 18 21:42:51 2019 +0100

    maybe this? no more stdlib flags

commit 2038f09
Author: Fytch <[email protected]>
Date:   Wed Dec 18 21:32:43 2019 +0100

    maybe this dist works?

commit a34655a
Author: Fytch <[email protected]>
Date:   Wed Dec 18 21:24:27 2019 +0100

    remove libc++ from clang 7,8,9 in travis

commit 02bf444
Author: Fytch <[email protected]>
Date:   Wed Dec 18 21:19:15 2019 +0100

    nicer build matrix

commit 5fc8044
Author: Fytch <[email protected]>
Date:   Wed Dec 18 21:11:01 2019 +0100

    add more compilers to travis

Author: Fytch

.travis.yml

@@ -1,7 +1,7 @@
 language: cpp
 
-dist: trusty
-sudo: required
+sudo: false
+dist: xenial
 
 notifications:
   email: false
@@ -15,39 +15,59 @@ matrix:
             - ubuntu-toolchain-r-test
           packages:
             - g++-4.9
-      env: COMPILER=g++-4.9
+      env:
+        - CC=gcc-4.9
+        - CXX=g++-4.9
     - compiler: gcc
       addons:
         apt:
           sources:
             - ubuntu-toolchain-r-test
           packages:
             - g++-5
-      env: COMPILER=g++-5
+      env:
+        - CC=gcc-5
+        - CXX=g++-5
     - compiler: gcc
       addons:
         apt:
           sources:
             - ubuntu-toolchain-r-test
           packages:
             - g++-6
-      env: COMPILER=g++-6
+      env:
+        - CC=gcc-6
+        - CXX=g++-6
     - compiler: gcc
       addons:
         apt:
           sources:
             - ubuntu-toolchain-r-test
           packages:
             - g++-7
-      env: COMPILER=g++-7
+      env:
+        - CC=gcc-7
+        - CXX=g++-7
     - compiler: gcc
       addons:
         apt:
           sources:
             - ubuntu-toolchain-r-test
           packages:
             - g++-8
-      env: COMPILER=g++-8
+      env:
+        - CC=gcc-8
+        - CXX=g++-8
+    - compiler: gcc
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-9
+      env:
+        - CC=gcc-9
+        - CXX=g++-9
     - compiler: clang
       addons:
         apt:
@@ -59,7 +79,8 @@ matrix:
             - libc++-dev
             - libc++abi-dev
       env:
-        - COMPILER=clang++-3.6
+        - CC=clang-3.6
+        - CXX=clang++-3.6
         - CMAKE_FLAGS="-DCMAKE_CXX_FLAGS=-stdlib=libc++ -DCMAKE_EXE_LINKER_FLAGS=-lc++"
     - compiler: clang
       addons:
@@ -72,7 +93,8 @@ matrix:
             - libc++-dev
             - libc++abi-dev
       env:
-        - COMPILER=clang++-3.7
+        - CC=clang-3.7
+        - CXX=clang++-3.7
         - CMAKE_FLAGS="-DCMAKE_CXX_FLAGS=-stdlib=libc++ -DCMAKE_EXE_LINKER_FLAGS=-lc++"
     - compiler: clang
       addons:
@@ -85,7 +107,8 @@ matrix:
             - libc++-dev
             - libc++abi-dev
       env:
-        - COMPILER=clang++-3.8
+        - CC=clang-3.8
+        - CXX=clang++-3.8
         - CMAKE_FLAGS="-DCMAKE_CXX_FLAGS=-stdlib=libc++ -DCMAKE_EXE_LINKER_FLAGS=-lc++"
     - compiler: clang
       addons:
@@ -98,7 +121,8 @@ matrix:
             - libc++-dev
             - libc++abi-dev
       env:
-        - COMPILER=clang++-3.9
+        - CC=clang-3.9
+        - CXX=clang++-3.9
         - CMAKE_FLAGS="-DCMAKE_CXX_FLAGS=-stdlib=libc++ -DCMAKE_EXE_LINKER_FLAGS=-lc++"
     - compiler: clang
       addons:
@@ -111,49 +135,85 @@ matrix:
             - libc++-dev
             - libc++abi-dev
       env:
-        - COMPILER=clang++-4.0
+        - CC=clang-4.0
+        - CXX=clang++-4.0
         - CMAKE_FLAGS="-DCMAKE_CXX_FLAGS=-stdlib=libc++ -DCMAKE_EXE_LINKER_FLAGS=-lc++"
     - compiler: clang
       addons:
         apt:
           sources:
             - ubuntu-toolchain-r-test
-            - llvm-toolchain-trusty-5.0
+            - sourceline: 'deb https://apt.llvm.org/xenial/ llvm-toolchain-xenial-5.0 main'
+              key_url: 'https://apt.llvm.org/llvm-snapshot.gpg.key'
           packages:
             - clang-5.0
-            - libc++-dev
-            - libc++abi-dev
       env:
-        - COMPILER=clang++-5.0
-        - CMAKE_FLAGS="-DCMAKE_CXX_FLAGS=-stdlib=libc++ -DCMAKE_EXE_LINKER_FLAGS=-lc++"
+        - CC=clang-5.0
+        - CXX=clang++-5.0
     - compiler: clang
       addons:
         apt:
           sources:
             - ubuntu-toolchain-r-test
-            - llvm-toolchain-trusty-6.0
+            - sourceline: 'deb https://apt.llvm.org/xenial/ llvm-toolchain-xenial-6.0 main'
+              key_url: 'https://apt.llvm.org/llvm-snapshot.gpg.key'
           packages:
             - clang-6.0
-            - libc++-dev
-            - libc++abi-dev
       env:
-        - COMPILER=clang++-6.0
-        - CMAKE_FLAGS="-DCMAKE_CXX_FLAGS=-stdlib=libc++ -DCMAKE_EXE_LINKER_FLAGS=-lc++"
+        - CC=clang-6.0
+        - CXX=clang++-6.0
+    - compiler: clang
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+            - sourceline: 'deb https://apt.llvm.org/xenial/ llvm-toolchain-xenial-7 main'
+              key_url: 'https://apt.llvm.org/llvm-snapshot.gpg.key'
+          packages:
+            - clang-7
+      env:
+        - CC=clang-7
+        - CXX=clang++-7
+    - compiler: clang
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+            - sourceline: 'deb https://apt.llvm.org/xenial/ llvm-toolchain-xenial-8 main'
+              key_url: 'https://apt.llvm.org/llvm-snapshot.gpg.key'
+          packages:
+            - clang-8
+      env:
+        - CC=clang-8
+        - CXX=clang++-8
+    - compiler: clang
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+            - sourceline: 'deb https://apt.llvm.org/xenial/ llvm-toolchain-xenial-9 main'
+              key_url: 'https://apt.llvm.org/llvm-snapshot.gpg.key'
+          packages:
+            - clang-9
+      env:
+        - CC=clang-9
+        - CXX=clang++-9
 
 script:
   - uname -a
   - cmake --version
-  - $COMPILER --version
+  - $CC --version
+  - $CXX --version
 
   - mkdir bin_noexcept
   - cd bin_noexcept
-  - cmake .. $CMAKE_FLAGS -DCMAKE_BUILD_TYPE=DEBUG -DCMAKE_CXX_COMPILER=$COMPILER -DPROGRAMOPTIONS_BUILD_TEST=OFF -DPROGRAMOPTIONS_NO_EXCEPTIONS=ON
+  - cmake .. $CMAKE_FLAGS -DCMAKE_BUILD_TYPE=DEBUG -DCMAKE_C_COMPILER=$CC -DCMAKE_CXX_COMPILER=$CXX -DPROGRAMOPTIONS_BUILD_TEST=OFF -DPROGRAMOPTIONS_NO_EXCEPTIONS=ON
   - cmake --build .
   - cd ..
 
   - mkdir bin
   - cd bin
-  - cmake .. $CMAKE_FLAGS -DCMAKE_BUILD_TYPE=DEBUG -DCMAKE_CXX_COMPILER=$COMPILER
+  - cmake .. $CMAKE_FLAGS -DCMAKE_BUILD_TYPE=DEBUG -DCMAKE_C_COMPILER=$CC -DCMAKE_CXX_COMPILER=$CXX
   - cmake --build .
 
   - ./Test