Reduce the degree of make parallelism in examples

And add a warning to not do this if it's not supported.

docs/osx/install.md

@@ -26,7 +26,10 @@ number of the processors in your system (which can be checked using `sysctl -n
 hw.ncpu` command if you are not sure), as this will dramatically speed up the
 build on modern systems. So in practice you should use a command like this:
 
-    make -j16
+    make -j8
+
+(but don't use it unless you actually have 8 CPUs and enough memory for that
+many parallel compiler invocations).
 
 You may also prefer to add `-s` option to avoid normal output from make and/or
 redirect it you to a log file for further inspection.