Add a build using GTK+ 3 to Travis

We need to test both wxGTK2 and wxGTK3 builds.

.travis.yml

@@ -14,7 +14,7 @@ matrix:
           compiler: gcc
         - dist: trusty
           compiler: gcc
-          env: wxCONFIGURE_FLAGS="--enable-cxx11 --enable-stl" wxMAKEFILE_FLAGS="CXXFLAGS=-std=c++11"
+          env: wxGTK_VERSION=3 wxCONFIGURE_FLAGS="--enable-cxx11 --enable-stl" wxMAKEFILE_FLAGS="CXXFLAGS=-std=c++11"
         - dist: trusty
           compiler: clang
           env: wxCONFIGURE_FLAGS="--disable-shared --disable-sys-libs"

build/tools/before_install.sh

@@ -10,7 +10,11 @@ case $(uname -s) in
     Linux)
         if [ -f /etc/apt/sources.list ]; then
             $SUDO apt-get update
-            $SUDO apt-get install -y libgtk2.0-dev libnotify-dev
+            case "$wxGTK_VERSION" in
+                3) libgtk_dev=libgtk-3-dev ;;
+                *) libgtk_dev=libgtk2.0-dev;;
+            esac
+            $SUDO apt-get install -y $libgtk_dev libnotify-dev
         fi
         ;;