I posted a bug (#136655) for libxml2 because it didn't compiled for me, but I realized that the problem was caused by confcache (because unmerging this caused this issue to solve) and I wanted to post a bug for confcache for this (after reading bug master for confcache). I will put all info requested for the failed compilation (with confcache reenabled and reinstalled) as attachments to this bug.
Created attachment 89049 [details] config.log for the failing libxml2 compilation this is the config,log for the libxml2 failing compile ebuild
Created attachment 89050 [details] confcache file /var/tmp/confcache/config.cache file
Created attachment 89051 [details] my emerge --info (with confcache reenabled)
Created attachment 89052 [details] genlop -l output genlop -l output results as requested in master bug for confcache.
I'm closing them as UPSTREAM as for now I can't do much until Brian provides me a new version of confcache that allows to trace who poisoned the cache. This is also the main reason why confcache is masked right now.