trying to build kmod ports without a src tree in /usr/src results in a failure that the build requires kernel source files in /usr/src. Mention also the make variable (SRC_BASE) that sets this patch as a hint to users building against a src tree located elsewhere.
My src trees are usually in a directory like ~/src/freebsd and the location can be set with SRC_BASE=..., but I can never recall if it's SRCTOP or SRC_TOP or whatnot. Mention the variable name in the error as a hint.