conflict with the kernel visible one. Make the new stuff not visible if
_ANSI_SOURCE or _POSIX_SOURCE is defined (!) because the new things are
not part of the base 1003.1-1990 standard (or ansi).
Define CLK_TCK only if _ANSI_SOURCE is not defined.
Don't include <machine/limits.h> to get the definition of CLK_TCK.
CLK_TCK should never have been defined there, and the inclusion
polluted the namespace.