Detect inotify.

Summary:
On Linux, inotify always exists; all you need is the header file.
On the BSDs, inotify is provided through a shim to kqueue, which
must be installed separately. Add a FindInotify to help sort
that out.

Based on RB 129316 and RB 129549.

Test Plan:

• On FreeBSD, reliably detects presence of libinotify in $LOCALBASE,
• Needs testing on Linux that it does find the header file.

Reviewers: apol, arrowdodger, Build System, Frameworks, tcberner, ervin, skelly, dfaure, kfunk

Reviewed By: tcberner, kfunk

Subscribers: kfunk, FreeBSD

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D3826