Removing duplicate AC_PREREQ macros

Setting AC_PREREQ required version to 2.50 until we have sorted out what is required. The manual says 2.58.

configure.in

-dnl -*- ksh -*-
+bdnl -*- ksh -*-
 dnl Process this file with autoconf to produce a configure script.
 
 AC_PREREQ(2.50)dnl		Minimum Autoconf version required.
@@ -116,8 +116,6 @@ AC_SUBST(SAVE_LDFLAGS)
 AC_SUBST(SAVE_CXXLDFLAGS)
 AC_SUBST(CXXLDFLAGS)
 
-AC_PREREQ(2.58)dnl		Minimum Autoconf version required.
-
 #AC_ARG_PROGRAM			# Automaticly invoked by AM_INIT_AUTOMAKE
 
 AM_SANITY_CHECK