fix ICONV_TRADITIONAL check for clang

necessary for OS X 10.8.2 XCode 4.6
vs_busy: use CLOCK_REALTIME instead of CLOCK_PROF

because:
- CLOCK_READTIME is a natural choice here.
- CLOCK_PROF seems less portable.
a fallback definition of clock_gettime

necessary for OSX
b64_ntop/pton is in not in libc on OSX