Travis: Only change CC and CXX for linux...

.travis.yml

@@ -25,7 +25,7 @@ addons:
     - cmake-data
 before_install:
   - $CC --version
-  - if [ "$CXX" = "g++" ]; then export CXX="g++-5" CC="gcc-5" EXTRA_LDFLAGS="-fuse-ld=gold"; else export CXX="clang++-3.8" CC="clang-3.8"; fi
+  - if [ "$TRAVIS_OS_NAME" == "linux" ]; then  if [ "$CXX" = "g++" ]; then export CXX="g++-5" CC="gcc-5" EXTRA_LDFLAGS="-fuse-ld=gold"; else export CXX="clang++-3.8" CC="clang-3.8"; fi; fi
   - $CC --version
   - cmake --version
 before_script: