config: add $egrep

Introduce a new variable set in $PLAN9/config:
$egrep, which is conditionally set to either
`egrep` or `grep -E`, based on platform, and use
this when we want to invoke `egrep`.

This works around the endless warnings from GNU
grep stating that `egrep` is deprecated in favor
of `grep -E`, when run on e.g. Linux.

INSTALL

@@ -65,11 +65,18 @@ DragonFly|*BSD)
 	echo "CFLAGS='-pthread'" >> $PLAN9/config
 	awk=awk
 	;;
+*Linux*)
+	awk=awk
+	egrep='grep -E'
+	;;
 *)
+	egrep=egrep
 	awk=awk
 	;;
 esac
 
+echo "egrep='$egrep'" >>$PLAN9/config
+
 (
 if [ `uname` = SunOS ]; then
 	# On Solaris x86, uname -p cannot be trusted.