The proposed solution is in the tracker. If this is an autotools based package, and '--disable-silent-rules' doesn't work, you can try to add MAKEOPTS+=" V=1"
Like in all other autototools-based packages, the best way to fix this is by updating the ebuild to EAPI5. Since libxml2 is keyworded basically everywhere, we can't do that until portage-2.1.11.31 has been stabilized on minority architectures (bug #440852).
The new libxml2-2.9.0 ebuild uses EAPI5. Hopefully by the time we want to stabilize it, a stable EAPI5 portage will be available for all arches.
libxml2-2.8.0-r4 is EAPI5 and has a verbose build log.