In 49af9a90, pocl LLVM passes were unconditionally set to compile with
-DNDEBUG. This causes kernel compile time asserts when LLVM is compiled with
assertions enabled:
llvm/lib/IR/Value.cpp:534: void llvm::ValueHandleBase::AddToExistingUseList(llvm::ValueHandleBase**): Assertion `VP.getPointer() == Next->VP.getPointer() && "Added to wrong list?"' failed.

Since llvm-config cannot be relied to pass the correct NDEBUG setting, this commit
adds a configure-time check to find out the correct value.