Building libreoffice with GCC 6 and LTO

New GCC is just around the corner. For me, as a GCC developer, this means a period of bugfixing, benchmarking and fine-tuning. Two years ago I wrote about my experiment  of building libreoffice with link time optimization (LTO). At that time LTO just got into shape of being able to build such large applications. Since that I am keeping my eye on this and try to be sure LTO keeps improving. I did not have time to publish any tests for GCC 5. Here is update for current trunk of GCC 6 compared to GCC 4.9.0, GCC 5.3, LLVM 3.5 and trunk.