Always install gcc-8 on travis, but exclude g++-8 for the C compilation tests.

.travis.yml

@@ -112,13 +112,11 @@ before_install:
   - |
     if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then
       sudo apt-get update
-      if [ "$BACKEND" = c ]; then
-        sudo apt-get install gcc-8
+      sudo apt-get install gcc-8  $(if [ "$BACKEND" = cpp ]; then echo -n "g++-8" )
       sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-8 60
       sudo update-alternatives --set gcc /usr/bin/gcc-8
       export CC=gcc-8
-      else
-        sudo apt-get install g++-8
+      if [ "$BACKEND" = cpp ]; then
         sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-8 60
         sudo update-alternatives --set g++ /usr/bin/g++-8
         export CXX=g++-8