Honor EUMM's DESTDIR option

[reyjrar]

Maybe I'm crazy, but I'm using cpanm inside of a mock chroot to build packages. The idea is to use Carton to package a script and it's dependencies as an RPM that can be deployed and run with carton exec. The first step is getting cpanminus to work inside the build environment. This patch will enable both cpanm and carton to correctly install and find their installed modules when the target install directory is the /build/builddir/<package-name>-<package-version> that exists while staging files for an RPM build.

[reyjrar]

I should add this differs from #583 as I'm trying to bootstrap a package building setup. I'm installing Perl into:

/opt/perl/530/bin/perl

And need to provide Carton to the build system and as a runtime requirement. The rest of the applications will be OK to use local::lib style directories, it's just Carton (possibly Carmel) that need to succeed the initial App::cpanminus bootstrap.

[Leont]

A better approach might be to use ExtUtils::InstallPaths with PERL_MB_OPT. That way you support all install path options in one go. Though in practice install_base and destdir are probably the only two options people actually use so it might not really matter.