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.

INSTALL

@@ -104,6 +104,7 @@ if [ `uname` = SunOS ]; then
 fi
 
 if [ `uname` = Darwin ]; then
+	export NPROC=$(sysctl hw.ncpu | sed 's/hw.ncpu: //')
 	# On Darwin, uname -m -p cannot be trusted.
 	echo "* Running on Darwin: checking architecture..."
 	rm -f ./a.out