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)