.travis.yml: attempt to support multiple GCC versions

Author: davidmalcolm

.travis.yml

@@ -1,7 +1,66 @@
+matrix:
+  include:
+    - os: linux
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-4.8 gcc-4.8-plugin-dev
+      env:
+         - MATRIX_EVAL="CC=gcc-4.8 && CXX=g++-4.8"
+
+    # works on Precise and Trusty
+    - os: linux
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-4.9 gcc-4.9-plugin-dev
+      env:
+         - MATRIX_EVAL="CC=gcc-4.9 && CXX=g++-4.9"
+
+    # works on Precise and Trusty
+    - os: linux
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-5 gcc-5-plugin-dev
+      env:
+         - MATRIX_EVAL="CC=gcc-5 && CXX=g++-5"
+
+    # works on Precise and Trusty
+    - os: linux
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-6 gcc-6-plugin-dev
+      env:
+        - MATRIX_EVAL="CC=gcc-6 && CXX=g++-6"
+
+    # works on Precise and Trusty
+    - os: linux
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-7 gcc-7-plugin-dev
+      env:
+        - MATRIX_EVAL="CC=gcc-7 && CXX=g++-7"
+
+before_install:
+    - eval "${MATRIX_EVAL}"
+
 language: c
 compiler:
   - gcc
 script: make
 before_install:
  - sudo apt-get update -qq
- - sudo apt-get install -qq gcc-4.8-plugin-dev python-six python-pygments graphviz python-lxml
+ - sudo apt-get install -qq python-six python-pygments graphviz python-lxml