Test builds with newer g++, clang and OS X on Travis CI

Configure the build matrix to test building on more platforms and using clang
as well and also test C++11 builds using both g++ and clang.

build/tools/before_install.sh

@@ -2,7 +2,7 @@
 #
 # This script is used by Travis CI to install the dependencies before building
 # wxWidgets but can also be run by hand if necessary but currently it only
-# works for Ubuntu 12.04 and 14.04 used by Travis.
+# works for Ubuntu 12.04 and 14.04 and OS X used by Travis builds.
 
 SUDO=sudo
 
@@ -13,4 +13,9 @@ case $(uname -s) in
             $SUDO apt-get install -y libcppunit-dev libgtk2.0-dev libnotify-dev
         fi
         ;;
+
+    Darwin)
+        brew update
+        brew install cppunit --universal
+        ;;
 esac