build them during the build stage and install them during fake.
devel/ruby-gems doesn't have separate build and install commands, as
most gems are pure ruby code and don't need a separate build stage.
When ruby-gems is installing a gem with C extensions, it builds them
during the install. Since installing is done during the fake stage,
this meant that the extensions were getting built as root.
Previously, this was required, as gem's --user-install option was
broken. However, since that option has now been fixed, we use
it to install the gem to a temporary location as the current
user during build, and then mv and chown the files during fake.
Thanks to bernd@ for pointing out that the fixed --user-install
option allowed this.
ok landry
8a77d5f794