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>
diff --git a/zebra/kernel_null.c b/zebra/kernel_null.c
index ec994a6..cdb6e23 100644
--- a/zebra/kernel_null.c
+++ b/zebra/kernel_null.c
@@ -16,7 +16,7 @@
 #endif
 
 int kernel_add_ipv6 (struct prefix *a, struct rib *b) { return 0; }
-#ifdef HAVE_SYS_WEAK_ALIAS_PRAGMA_PRAGMA
+#ifdef HAVE_SYS_WEAK_ALIAS_PRAGMA
 #pragma weak kernel_delete_ipv6 = kernel_add_ipv6
 #else
 int kernel_delete_ipv6 (struct prefix *a, struct rib *b) { return 0; }