adding --extra option and correction pathes to ispc compiler
Merge pull request #623 from ifilippov/testing

adding --extra option and correction pathes to ispc compiler
Switching default compiler on Unix from g++ to clang++
Switch examples on Unix from using g++ to clang++
Fix problem with building ISPC by clang 3.4
Enable clang as default C++ compiler used by run_tests.py and perf.py
fail_db.txt update with fails with clang 3.3 as a C++ compiler for tests (Linux)