lib/replace/wscript: fix detection of a fallthrough attribute for clang

lib/replace/wscript: fix detection of a fallthrough attribute for clang

clang issues a warning but otherwise allows our test to be compiled and
linked. We consider this case a successful pass for a fallthrough
attribute detection while it is an error.

Turn missing declaration warning into an error so that we actually do
not define fallthrough attribute support in case it doesn't really work.

Fixes FreeBSD 11.2 with clang.

Signed-off-by: Alexander Bokovoy <ab@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>