http://src.opensolaris.org/source/xref/sfw/usr/src/cmd/bison/bison-1.875/lib/timevar.c

     79 /* Calculation of scale factor to convert ticks to microseconds.
     80    We mustn't use CLOCKS_PER_SEC except with clock().  */
     81 #if HAVE_SYSCONF && defined _SC_CLK_TCK
     82 # define TICKS_PER_SECOND sysconf (_SC_CLK_TCK) /* POSIX 1003.1-1996 */
     83 #else
     84 # ifdef CLK_TCK
     85 #  define TICKS_PER_SECOND CLK_TCK /* POSIX 1003.1-1988; obsolescent */
     86 # else
     87 #  ifdef HZ
     88 #   define TICKS_PER_SECOND HZ  /* traditional UNIX */
     89 #  else
     90 #   define TICKS_PER_SECOND 100 /* often the correct value */
     91 #  endif
     92 # endif
     93 #endif