LWS_WITH_ASAN

This lets you build using the runtime Address Sanitizer in gcc.

LWS is heavily tested with valgrind routinely during development.  But ASAN
did find some theoretical-only issues with shifting, strictly ~(1 << 31) is
a signed int, it should be ~(1u << 31).  Gcc does the same for both, but it's
good to have the ability to find these.