Some packages using Libtool have the possibility
of using this environment variable if set,
instead of the direct command for "file" using PATH,
or the need to patch a hard-coded path.

This is a new feature of Libtool 2.4.7

Ref: bf261073 ("tools/libtool: bump to 2.4.7")
Signed-off-by: Michael Pratt <mcpratt@pm.me>