gem install nokogiri -- --use-system-libraries \
--with-xml2-dir=/usr \
--with-xslt-dir=/usr \
--with-exslt-dir=/usr
I can't say I understand why I had to specify those dirs as that's the system default, but it works and I'm in a hurry. I'll update this as I learn more someday.
Meanwhile, you may be interested in the Nokogiri issue that this stems from