libwww doesn't compile with flags -DNDEBUG -DG_DISABLE_ASSERT in CFLAGS. Hire is patch on ebuild that filter this flags
Created attachment 31010 [details, diff] patch on ebuild
Thanks for the patch. Actually -DNDEBUG causes the failure and clearing CFLAGS is not enough (you might add -DNDEBUG to CPPFLAGS, for example) so I made a patch to disable -DNDEBUG completely.