Update to work with autoconf 2.71

The script needs CPP so add the appropriate macro.

Upstream-Status: Inappropriate [needed for old GPLv2 version only]
RP - 2021/1/28

Index: grep-2.5.1a/configure.in
===================================================================
--- grep-2.5.1a.orig/configure.in
+++ grep-2.5.1a/configure.in
@@ -27,6 +27,7 @@ AC_PROG_AWK
 AC_PROG_CC
 AC_PROG_INSTALL
 AC_PROG_RANLIB
+AC_PROG_CPP
 
 dnl Checks for typedefs, structures, and compiler characteristics.
 AC_SYS_LARGEFILE