INSTALL: set $NPROC on macOS

My MacBook Pro has hw.ncpu=12, so set NPROC=12.
Speeds up INSTALL dramatically.
Should probably add similar code to other OSes.

Also silence rio warning from earlier commit.

dist/isum.awk

@@ -132,7 +132,7 @@ errors != 0 {
 	next
 }
 
-/(up to date|nothing to see|assuming it will be|loop not entered)/ {
+/(up to date|nothing to see|assuming it will be|loop not entered|# WSYSTYPE)/ {
 	next
 }