build: fix "pragma weak" mixups

build: fix "pragma weak" mixups

Not only was there a minor typo in the "pragma weak" preprocessor
checks, but also were the tests not behaving as needed - they only
indicated support for the /first/ method of implementing weak aliases,
which on Linux is __attribute__ and not #pragma.

* m4/ax_sys_weak_alias.m4: set defines for _all_ weak alias methods
* zebra/kernel_null.c: fix typo

Cc: Doug VanLeuven <roamdad@sonic.net>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>