app-editors/emacs-vcs: Remove sandbox hack in src_compile.

app-editors/emacs-vcs: Remove sandbox hack in src_compile.

Configure with explicit option --with-dumping=pdumper to make sure
that the portable dumper is used, even for the (unlikely) case that
the upstream default would change.

Note that (as of today) --with-dumping=unexec breaks the build
process, therefore controlling this option with a USE flag wouldn't
make sense.

Closes: https://bugs.gentoo.org/700182
Package-Manager: Portage-2.3.79, Repoman-2.3.18
Signed-off-by: Ulrich Müller <ulm@gentoo.org>