ci: enable compilation with "-Werror"

During the conversion of our CI scripts in bf418f09 (ci: refactor unix
ci build/test scripts, 2018-07-14), we accidentally dropped the
"-DENABLE_WERROR=ON" switch in our cmake invocation. Re-add it to help
us catch compiler warnings early.

ci/build.sh

@@ -29,7 +29,7 @@ echo "## Configuring build environment"
 echo "##############################################################################"
 
 echo cmake ${SOURCE_DIR} -DBUILD_EXAMPLES=ON ${CMAKE_OPTIONS}
-cmake ${SOURCE_DIR} -DBUILD_EXAMPLES=ON -DBUILD_FUZZERS=ON -DUSE_STANDALONE_FUZZERS=ON ${CMAKE_OPTIONS}
+cmake ${SOURCE_DIR} -DENABLE_WERROR=ON -DBUILD_EXAMPLES=ON -DBUILD_FUZZERS=ON -DUSE_STANDALONE_FUZZERS=ON ${CMAKE_OPTIONS}
 
 echo ""
 echo "##############################################################################"