A common scenario is for a developer to have two configurations, a product and a debug version, e.g. linux-x64 and linux-x64-debug. To select the latter using CONF is just a matter of using CONF=debug, but to select the former, the complete name needs to be supplied: CONF=linux-x64.

Instead, this patch introduces the use of "!" as a negation character, so CONF=!debug would select all configurations that do not match debug.