On Mon, 2016-11-07 at 12:26 -0500, Jeff King wrote: > I have in the back of my mind a fear that it is harder to unset a > make variable than it is to override it with a new value (which is > what you'd want to do here to turn openssl back on), It depends on what you mean by "unset". If you mean it as per the shell "unset" command, where the variable is completely forgotten as if it never was set at all, that's tricky. You have to use the "undefine" special command which was introduced in GNU make 3.82 (released in 2010). But if you just want to set the variable to the empty string, using "FOO=" works fine for that in all versions of make (GNU and otherwise) and using all the normal rules (command line override, etc.) It's not easy to distinguish between a variable that is empty and one that is actually not defined, in make, so it's a difference without a distinction in almost all situations.