Add static-only build to Travis test matrix

Author: nwellnhof

.travis.yml

@@ -8,6 +8,11 @@ language: c
 compiler:
  - clang
  - gcc
+matrix:
+  include:
+  - os: linux
+    compiler: gcc
+    env: CMAKE_OPTIONS="-DCMARK_SHARED=OFF"
 addons:
   apt:
     # we need a more recent cmake than travis/linux provides (at least 2.8.9):
@@ -26,7 +31,7 @@ before_install:
      fi
 
 script:
- - make
+ - (mkdir -p build && cd build && cmake $CMAKE_OPTIONS ..)
  - make test
  - |
      if [ ${TRAVIS_OS_NAME:-'linux'} = 'linux' ]