Fix building with --enable-async-push in FreeBSD

Fix building with --enable-async-push in FreeBSD

This option can be used in FreedBSD with devel/libinotify installed.

Detect presence of libinotify with pkgconf and use its word
to compile and link.

Trac: #1256

Signed-off-by: Lev Stipakov <lstipakov@gmail.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20200314052906.28095-1-lstipakov@gmail.com>
URL: https://www.mail-archive.com/search?l=mid&q=20200314052906.28095-1-lstipakov@gmail.com
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit e1eb630df1b3ef5df88afd51003fbec40e5d6d67)