netlink's API is stable, and strictly defined by the integer values that make
up commands and attributes. There is little reason do disable a netlink feature
based on compile time detection of the kernel headers.

Either kernel supports it, or it will fail with an appropriate response.

Also, support for NL80211_CMD_CRIT_PROTOCOL_START was merge to kernel
in 2013. Maybe, we should now just always assume support (in the kernel
headers is there). Anyway, don't do that yet, but instead avoid the
defines and use the numeric values directly.