Fix ICONV_TRADITIONAL check for clang

necessary for OS X 10.8.2 XCode 4.6

edit: gcc42 breaks under -Wno-dangling-else; make it clang-only.
b64_ntop/pton is in not in libc on OSX
vs_busy: use CLOCK_REALTIME instead of CLOCK_PROF

because:
- CLOCK_READTIME is a natural choice here.
- CLOCK_PROF seems less portable.