When invoked in projects that don't set CMP0054 to NEW, FindOpenMP
triggers a warning:

    CMake Warning (dev) at .../Modules/FindOpenMP.cmake:... (if):
      Policy CMP0054 is not set...Quoted variables like "c" will no
      longer be dereferenced when the policy is set to NEW.  Since
      the policy is not set the OLD behavior will be used.

Set the policy within the module's scope to avoid this.

Fixes: #17292