meson: Add yajl kludge

meson: Add yajl kludge

If this looks familiar, that's because it's literally *the
same code* that we used to work around *the same issue* in
readline before 1635dca26f61def3fbf56c70fbbfe514f2b50987 :)

Note that the issue only really affects people building from
source on Apple Silicon: on Intel, Homebrew installs header
files under directories that are part of the default search
path, which explains why our CI pipeline never ran into it.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Roman Bolshakov <r.bolshakov@yadro.com>
Tested-by: Roman Bolshakov <r.bolshakov@yadro.com>