travis: export CXX and CC

.travis.yml

@@ -20,7 +20,8 @@ env:
 before_install:
   - sudo add-apt-repository -y ppa:ubuntu-toolchain-r/test
   - sudo apt-get -qq update
-  - sudo apt-get -qq install g++-4.8
+  - if [ "$CXX" = "g++" ]; then sudo apt-get install -qq g++-4.8; fi
+  - if [ "$CXX" = "g++" ]; then export CXX="g++-4.8" CC="gcc-4.8"; fi
 install:
   - sudo apt-get install libcomedi-dev libcommoncpp2-dev libmysqlclient-dev libomniorb4-dev omniidl libpqxx3-dev librrd-dev libsigc++-2.0-dev libsqlite3-dev python-dev