The build of 7.8 that worked was with gcc-6 6.2.0-4, which according to the changelog is actually "SVN 20160914 (r240133, 6.2.1) from the gcc-6-branch." The build of 7.8 that failed was with 6.2.0-11, which is "SVN 20161103 (r241817, 6.2.1) from the gcc-6-branch".