configure.ac: tweak inotify check

Our check for inotify_init1() being defined is broken.  We happily
declare that inotify is supported, even if the check fails.

This was originally intended to check for inotify_init1 in the libc so
that we could fall back to inotify_init if it was not yet defined.

FreeBSD has a libinotify that emulates the inotify API via kqueue.  It
installs a <sys/inotify.h> header and requires linking to -linotify.  We
don't want to falsely detect working inotify in this case.

Treat the lack of inotify_init1() in the libc as a lack of inotify
support.  This requires only a new libc -- we still support old kernels:
in the case that inotify1_init() fails, we fall back to inotify_init().

https://bugzilla.gnome.org/show_bug.cgi?id=724330