Only try to install GCC 4.8 on Linux.

travis.sh

@@ -11,12 +11,14 @@
 # For when some other test needs the C++ main build, including protoc and
 # libprotobuf.
 internal_build_cpp() {
-  # Install GCC 4.8 to replace the default GCC 4.6. We need 4.8 for more
-  # decent C++ 11 support in order to compile conformance tests.
-  sudo add-apt-repository ppa:ubuntu-toolchain-r/test -y
-  sudo apt-get update -qq
-  sudo apt-get install -qq g++-4.8
-  export CXX="g++-4.8" CC="gcc-4.8"
+  if [ $(uname -s) == "Linux" ]; then
+    # Install GCC 4.8 to replace the default GCC 4.6. We need 4.8 for more
+    # decent C++ 11 support in order to compile conformance tests.
+    sudo add-apt-repository ppa:ubuntu-toolchain-r/test -y
+    sudo apt-get update -qq
+    sudo apt-get install -qq g++-4.8
+    export CXX="g++-4.8" CC="gcc-4.8"
+  fi
 
   ./autogen.sh
   ./configure