maint.mk: catch more abuse of HAVE_DECL in syntax-check

maint.mk: catch more abuse of HAVE_DECL in syntax-check

Libvirt had a patch that attempted to do:

+#elif defined(SIOCSIFLLADDR) && defined(HAVE_STRUCT_IFREQ) && \
+    defined(HAVE_DECL_LINK_ADDR)

but which was not flagged by the syntax checker as suspicious
(all HAVE_DECL_* symbols defined by autoconf are always defined
after including <config.h>, although they are sometimes defined
as the value 0).  Now that code is flagged until changed to:

+#elif defined(SIOCSIFLLADDR) && defined(HAVE_STRUCT_IFREQ) && \
+    HAVE_DECL_LINK_ADDR

* top/maint.mk (sc_prohibit_defined_have_decl_tests): Relax regex.

Signed-off-by: Eric Blake <eblake@redhat.com>