(Re-)try really hard to install gcc-8 and g++-8 in travis, even if it fails a couple of times.

.travis.yml

@@ -13,8 +13,8 @@ addons:
       - python-dbg
       - python3-dbg
       - libzmq-dev  # needed by IPython/Tornado
-      - gcc-8
-      - g++-8
+      #- gcc-8
+      #- g++-8
 
 cache:
   pip: true
@@ -37,7 +37,7 @@ env:
     - CCACHE_SLOPPINESS=pch_defines,time_macros
     - CCACHE_COMPRESS=1
     - CCACHE_MAXSIZE=150M
-    - PATH="/usr/lib/ccache:$HOME/gcclinks:$HOME/miniconda/bin:$PATH"
+    - PATH="/usr/lib/ccache:$HOME/miniconda/bin:$PATH"
   matrix:
     - BACKEND=c
     - BACKEND=cpp
@@ -116,9 +116,13 @@ branches:
 before_install:
   - |
     if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then
-      mkdir -p $HOME/gcclinks
-      ln -s /usr/bin/gcc-8 $HOME/gcclinks/gcc
-      ln -s /usr/bin/g++-8 $HOME/gcclinks/g++
+      # adding apt repos in travis is really fragile => retry a couple of times.
+      sudo apt-add-respository 'ppa:ubuntu-toolchain-r/test'
+      for i in {1..10}; do sudo apt-get update && break; done
+      for i in {1..10}; do sudo apt-get install gcc-8  $(if [ "$BACKEND" = cpp ]; then echo -n "g++-8"; fi ) && break ; done
+      sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-8 60 --slave /usr/bin/g++ g++ /usr/bin/g++-8
+      sudo update-alternatives --set gcc /usr/bin/gcc-8
+      sudo update-alternatives --set g++ /usr/bin/g++-8
       export  CC=gcc-8  CXX=g++-8
     fi