configure.in: require at least autoconf 2.54

AC_PROG_EGREP requires autoconf 2.54, so bump up the AC_PREREQ
accordingly.

Reported-by: g.esp@free.fr
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
diff --git a/configure.in b/configure.in
index 1099309..cf96e43 100644
--- a/configure.in
+++ b/configure.in
@@ -1,5 +1,5 @@
 AC_INIT(version.h)
-AC_PREREQ(2.50)
+AC_PREREQ(2.54)
 AC_CONFIG_AUX_DIR(config)
 AC_CONFIG_HEADERS([lib/config.h])
 AH_BOTTOM([#include "dirpaths.h"])