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.

bin/9l

@@ -146,7 +146,7 @@ then
 					grep '__p9l_autolib_[a-zA-Z0-9+-]*' |
 					sed 's/.*__p9l_autolib_//; s/:.*//' |
 					sort -u |
-					egrep -v '^(thread|draw)$'
+					$egrep -v '^(thread|draw)$'
 				`
 				a="$a $b"
 			done
@@ -337,7 +337,7 @@ quiet()
 	ignore=$ignore'|ld: warning: ignoring duplicate libraries:'
 
 	sed 's/.*: In function `[^:]*: *//' "$1" |
-	egrep -v "$ignore"
+	$egrep -v "$ignore"
 }
 
 # Must use temp file to avoid pipe; pipe loses status.