Makefile: fix build with make 3.80 again

According to Documentation/Changes, make 3.80 is still being supported
for building the kernel, hence make files must not make (unconditional)
use of features introduced only in newer versions.  Commit 8779657d29c0
("stackprotector: Introduce CONFIG_CC_STACKPROTECTOR_STRONG") however
introduced an "else ifdef" construct which make 3.80 doesn't understand.

Also correct a warning message still referencing the old config option
name.

Apart from that I question the use of "ifdef" here (but it was used that
way already prior to said commit): ifeq (,y) would seem more to the
point.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Michal Marek <mmarek@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>