tests: Only configure tests when asked for

This prevents the download of googletest for normal builds. Now when we
want to compile and run tests we need to specify -DBUILD_TESTS=ON

.travis.yml

@@ -23,7 +23,7 @@ matrix:
       env: BUILD_TYPE=Debug
 
     - compiler: gcc
-      env: BUILD_TYPE=Debug POLYBAR_BUILD_TYPE=tests
+      env: BUILD_TYPE=Debug POLYBAR_BUILD_TYPE=tests BUILD_TESTS=ON
       before_install:
         - pip install --user cpp-coveralls
       script: ${TRAVIS_BUILD_DIR}/common/travis/tests.sh

CMakeLists.txt

@@ -28,9 +28,7 @@ add_subdirectory(src bin)
 
 # We need to enable testing in the root folder so that 'ctest' and 'make test'
 # can be run in the build directory
-enable_testing()
 if(BUILD_TESTS)
-  add_subdirectory(tests)
-else()
+  enable_testing()
   add_subdirectory(tests EXCLUDE_FROM_ALL)
 endif()