add documentation for upgrading gcc and clang

user/languages/cpp.md

@@ -93,3 +93,134 @@ before_install:
   - test -n $CC  && unset CC
   - test -n $CXX && unset CXX
 ```
+
+### Modern C++ and Compiler Versioning
+
+If you need a more modern toolchain or more granular control over your tools, you will have to handle that yourself. Fortunately, the community has already whitelisted the [repositories](https://github.com/travis-ci/apt-source-whitelist/blob/master/ubuntu.json) and [packages](https://github.com/travis-ci/apt-package-whitelist/blob/master/ubuntu-precise) necessary for such upgrades.
+
+#### GCC on Linux
+
+Ubuntu 12.04 shipped with GCC 4.6.3 and Ubuntu 14.04 shipped with GCC 4.8.2. If you need a more recent version (if you are working with C++11, you *must* do this; support for C++11 was not finished until GCC 4.9), you will want to follow these directions:
+
+```yaml
+matrix:
+  include:
+    - os: linux
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-4.9
+      env:
+         - MATRIX_EVAL="CXX=g++-4.9"
+
+    - os: linux
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-5
+      env:
+         - MATRIX_EVAL="CXX=g++-5"
+
+    - os: linux
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-6
+      env:
+        - MATRIX_EVAL="CXX=g++-6"
+
+before_install:
+    - eval "${MATRIX_EVAL}"
+```
+
+#### GCC on OSX
+
+```yaml
+matrix:
+  include:
+    - os: osx
+      osx_image: xcode8
+      env:
+        - MATRIX_EVAL="CC=gcc-4.9 && CXX=g++-4.9"
+
+    - os: osx
+      osx_image: xcode8
+      env:
+        - MATRIX_EVAL="brew install gcc5 && CC=gcc-5 && CXX=g++-5"
+
+    - os: osx
+      osx_image: xcode8
+      env:
+        - MATRIX_EVAL="brew install gcc && CC=gcc-6 && CXX=g++-6"
+
+before_install:
+    - eval "${MATRIX_EVAL}"
+```
+
+#### LLVM
+
+Be warned that the LLVM-Travis ecosystem [has faced some stability problems for over a year now](https://github.com/travis-ci/apt-source-whitelist/issues/156).
+
+```yaml
+matrix:
+  include:
+    - os: linux
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+            - llvm-toolchain-precise-3.6
+          packages:
+            - clang-3.6
+      env:
+        - MATRIX_EVAL="CC=clang-3.6 && CXX=clang++-3.6"
+
+    - os: linux
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+            - llvm-toolchain-precise-3.7
+          packages:
+            - clang-3.7
+      env:
+        - MATRIX_EVAL="CC=clang-3.7 && CXX=clang++-3.7"
+
+    - os: linux
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+            - llvm-toolchain-precise-3.8
+          packages:
+            - clang-3.8
+      env:
+        - MATRIX_EVAL="CC=clang-3.8 && CXX=clang++-3.8"
+
+before_install:
+    - eval "${MATRIX_EVAL}"
+```
+
+On OSX, the version of `clang` is controlled by the choice of `osx_image`.
+
+#### CMake
+
+To upgrade `cmake` (note that the most recent verison of `cmake` that has been backported is 3.2), add the following to each `apt` addon which requires it:
+
+```yaml
+addons:
+  apt:
+    sources:
+      - george-edison55-precise-backports
+    packages:
+      - cmake-data
+      - cmake
+```
+
+On OSX, the version of `cmake` is controlled by the choice of `osx_image`.