Incomplete meson port cannot disable fdatasync use

Downstream users of shared-mime-info often disable the anti-feature of running fdatasync() on unimportant files outside of the package manager controlled facility to run sync triggers site-wide.

Original shared-mime-info bug: https://bugs.freedesktop.org/show_bug.cgi?id=70366

Downstream bugs: https://bugs.gentoo.org/487504 https://bugs.archlinux.org/task/38836

Tracking bug for the reintroduction of this since the meson port: https://bugs.archlinux.org/task/66687

Please add a meson_options.txt switch to disable this, to make it easier to control. This regression is causing grief to our users.