In commit dc688857 (Pass EXCLUDE_FROM_ALL from directory to targets,
2019-01-15, v3.14.0-rc1~83^2) all `AddNewTarget` call sites were updated
to copy the directory-level `EXCLUDE_FROM_ALL` into the target property
of the same name, except that the one for `include_external_msproject`
was incorrectly missed.  Add it now.

Furthermore, refactoring in commit b99129d2 (ENH: some code cleanup,
2007-03-12, v2.6.0~2020) accidentally set the `EXCLUDE_FROM_ALL` target
property of `include_external_msproject`-generated targets to `FALSE`
instead of simply leaving it unset.  This was not necessary but had no
effect until the above commit gave it a meaning.  Drop that.

Fixes: #18986