Use config.h more

Summary:
This also fixes an issue with regards to HAVE_FDATASYNC:

src/buffer/katesecuretextbuffer.cpp:157:5:
warning: 'HAVE_FDATASYNC' is not defined, evaluates to 0 [-Wundef]
#if HAVE_FDATASYNC

^

> The code path with fdatasync() was never enabled in that file prior

to this patch

Reviewers: cullmann

Reviewed By: cullmann

Subscribers: cullmann, kwrite-devel, Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D7459