Revert "Replaced fixed -j parm with call to nproc"

Not everyone has `nproc`. In particular it does not appear to be present on my OSX build. Better to leave a fixed number here. This is just a suggestion anyhow.

This reverts commit 9c18f2ad.

c++/setup-autotools.sh

@@ -3,4 +3,4 @@
 set -euo pipefail
 
 echo "This script is no longer needed. Go ahead and run autoreconf. For example:"
-echo "  autoreconf -i && ./configure && make -j`nproc` check && sudo make install"
+echo "  autoreconf -i && ./configure && make -j6 check && sudo make install"