travis: try out build matrix

.travis.yml

@@ -2,16 +2,16 @@ language: c
 sudo: required
 dist: trusty
 
-env:
-- CFLAGS=-Wall -W -Werror
-
-os:
-  - linux
-  - osx
-
-compiler:
-  - gcc
-  - clang
+matrix:
+    include:
+        - os: linux
+        env: CC=gcc
+        - os: linux
+        env: CC=clang
+        - os: osx
+        env: CC=gcc
+        - os: osx
+        env: CC=clang
 
 cache:
   apt:  true
@@ -34,6 +34,7 @@ before_script:
   - mkdir build
   - cd build
   - cmake ..
+  - set CFLAGS=-Wall -W -Werror
 
 before_install:
   - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew update ; fi