Unset CFLAGS/CXXFLAGS when running tests via cmake/make check targets
- should fix automatic tests set up on http://llvm-jenkins.debian.net
- needed in order to make Debian package builds depend on passing test suite



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@181736 91177308-0d34-0410-b5e6-96231b3b80d8