include_external_msproject: Restore support for EXCLUDE_FROM_ALL
In !2816 (merged) 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 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 !2816 (merged) gave it a meaning. Drop that.
Fixes: #18986 (closed)