travis: add clang support

.travis.yml

@@ -1,8 +1,15 @@
 language: cpp
+compiler:
+  - gcc
+  - clang
 env:
   - SUBTARGET=arcade
   - SUBTARGET=mess
-script: make -j2
+script:
+  - if [ $CC == 'clang' ];
+    then make -j2 linux_x64_clang;
+    else make -j2;
+    fi
 sudo: required
 before_install:
   - sudo add-apt-repository ppa:zoogie/sdl2-snapshots -y