scram-project-build.file: fix processing of debug symbols

  - adapt to the way scram is handling multiple USER_CXXFLAGS
  - fix dwz options in order to find the shared debug symbols after
    files are moved under .debug/
Merge pull request #727 from fwyzard/IB/CMSSW_7_1_X/debug

scram-project-build.file: fix processing of debug symbols