Double quote to prevent globbing and word splitting.

Author: m-kuhn

.ci/travis/linux/docker-build-test.sh

@@ -56,7 +56,7 @@ cmake \
  -DWITH_SERVER=ON \
  -DDISABLE_DEPRECATED=ON \
  -DPYTHON_TEST_WRAPPER="timeout -sSIGSEGV 55s"\
- -DCXX_EXTRA_FLAGS=${CLANG_WARNINGS} ..
+ -DCXX_EXTRA_FLAGS="${CLANG_WARNINGS}" ..
 echo "travis_fold:end:cmake"
 
 #######