Band-aid to let lib/libmagic compile on older branches

[dim]

Summary

Sicne the MFC of file 5.21 in r276416, stable/9 cannot be built from any
FreeBSD 8.x version. This is because the build-tools stage requires
libmagic, but lib/libmagic/config.h was generated on -current, and it
now enables using the xlocale.h APIs, which are not supported on 8.x
(and on 9.x before __FreeBSD_version 900506).

See also the start of this thread on -stable:

https://lists.freebsd.org/pipermail/freebsd-stable/2015-January/081521.html

Applying this diff directly to stable/9 makes it build on 8.4-RELEASE.
Let's first get it into -current, then MFC it after a suitable timeout.

Test Plan

Build world (or just the build-tools stage) from a few older versions of
FreeBSD, e.g 8.x building stable/9, 9.0-RELEASE building stable/9.

[delphij]

Looks fine to me.

[dim]

Closed by commit rS277147 (authored by @dim).