Set x_static_install 1 if it is appropriate

[skaji]

Now milla sets x_static_install 1 if it is appropriate. https://metacpan.org/release/MIYAGAWA/Dist-Milla-v1.0.19

I am happy if Minilla follows that.

In this PR: If static_install = "auto" is specified in minil.toml (or nothing is specified), then minil tries to detect whether distributions are ready for static install or not, and set x_static_install accordingly. If users specify static_install = 0/1 explicitly, then minil will respect it.

@tokuhirom Feel free to reject this PR.

See also

• The spec of static install: https://github.com/Perl-Toolchain-Gang/cpan-static
• A Dist::Zilla plugin for static install: https://metacpan.org/pod/Dist::Zilla::Plugin::StaticInstall