pistachio: Explicitly require autoconf >= 2.50

'make -C base-pistachio prepare' produces the following error otherwise

  autoconf: Undefined macros:
  configure.in:117:AC_CONFIG_HEADERS([config.h])
  configure.in:129:Please[AC_HELP_STRING([--with-comport=PORT],
  configure.in:138:Please[AC_HELP_STRING([--with-comspeed=SPEED],
  configure.in:147:Please[AC_HELP_STRING([--with-kickstart-linkbase=BASE],
  configure.in:152:Please[AC_HELP_STRING([--with-s0-linkbase=BASE],
  configure.in:157:Please[AC_HELP_STRING([--with-roottask-linkbase=BASE],
  configure.in:162:Please[AC_HELP_STRING([--with-kerneldir=DIR],
  configure.in:341:AC_CONFIG_FILES([
  configure.in:36:AC_CONFIG_SRCDIR([serv/sigma0/sigma0.cc])

Fixes #1034.