Fixes noise in the CI build:

    CMake Warning (dev) at /builds/gitlab-kitware-sciviz-ci/build/CTestCustom.cmake:51 (set):
      Syntax error in cmake code at
        /builds/gitlab-kitware-sciviz-ci/build/CTestCustom.cmake:60
      when parsing string
        plugin/.*Plugin\.h
      Invalid escape sequence \.
      Policy CMP0010 is not set: Bad variable reference syntax is an error.  Run
      "cmake --help-policy CMP0010" for policy details.  Use the cmake_policy
      command to set the policy and suppress this warning.
    This warning is for project developers.  Use -Wno-dev to suppress it.