Merge branch/2013-08-21/lii6ll/... into the master sources, adding new supported platform LII6LL (Linux, x86-64, Clang/LLVM).
Configure Travis to build using both GCC and Clang.
Use macro NONNEGATIVE so that we can keep asserting that values are >= 0 without provoking "unsigned comparison >= 0 is always true" warnings from GCC.