Warn that option `-preprocess` is ignored when used with option `-clang`
git-svn-id: https://svn.sosy-lab.org/software/cpachecker/branches/872-clang-warn-when-used-with-option-preprocess@37063 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c
Closes #872 (closed)