mkrf_conf_xapian.rb: Fallback to Gem.user_dir

[IPv2]

ext/mkrf_conf_xapian.rb didn't check writability of the requested gem installation directory before attempting to install the xapian-ruby gem.

In typical default configurations, this would cause installation of the sup gem to fail for non-root users (because of no write permission by non-root users to the default system-wide gem directory).

Examples of gem installation workarounds that were being used due to this:

• Run gem install sup as root user. Not recommended for many distributions due to potential conflicts with the distribution's own package manager.

• Manually install the xapian-ruby gem first before attempting to install the sup gem. Not ideal, because it adds a manual step to what should be an automatically-installed dependency.

• Explicitly set the GEM_HOME environment variable before calling gem install. Not ideal, because this shouldn't be required.

In this commit, we modify ext/mkrf_conf_xapian.rb to:

1. Check that the requested gem installation directory is writable. If not, then fallback to installing xapian-ruby into the user gem directory (Gem.user_dir) instead.

2. Improve error handling: output a useful error message to STDERR if the attempted installation of xapian-ruby fails for any reason.

[danc86]

I don't really like that we have this weird fake extension and that it tries to implicitly install stuff using gem install... in the Fedora package this is just patched out because xapian bindings are not installed as a gem.

But anyway, I get that this is just providing a more convenient installation method for people who aren't getting xapian from their distro packages so this seems like a nice improvement.

[IPv2]

That's an interesting approach in Fedora - and I wonder if there is a possible further improvement we could make.

In Debian-based distributions, the packaged version of Xapian (Debian package ruby-xapian) isn't installed as a gem.

This means that in mkrf_conf_xapian.rb, the existing check doesn't detect that the distro package is installed - as it is explicitly looking for a gem:

# try to load gem
gem name, version

Perhaps in a future PR, we should modify this check to instead (say) try to require "xapian" - which would detect the existing installation.