AC_ARG_ENABLE([entropy], 
  AS_HELP_STRING([--disable-entropy], [disables kernel entropy device]))
[ if x"$enable_entropy" != xno ]; then]
  AC_DEFINE([MACH_ENTROPY], [0], [Enable entropy device?])
  AM_CONDITIONAL([enable_entropy], [false])
[else]
  AC_DEFINE([MACH_ENTROPY], [1], [Enable entropy device?])
  AM_CONDITIONAL([enable_entropy], [true])
[fi]