Commits on Jan 5, 2017

1. Disables link-time optimization unless -DENABLE_LTO=ON is passed. …

   This disables the `-flto` LTO flag by default since we're seeing
   segfaults in compiler lto plugins, binutils and linker errors again and
   again for various clang / gcc / binutils combinations.
   
   Pass `-DNEBALE_LTO` to `cmake` in order to re-enable LTO.
   
   LTO situation in short:
   - LTO does not work at all for gcc<4.9
   - With gcc>=4.9 the "slim" LTO format is getting used dumping IR
   - Older binutils need LTO plugins which know how to read this IR
   - Recent binutils handle this format all by themselves
   - LLVM is more or less the same with some Clang versions segfaulting
   
   If you need the performance benefit of LTO, make sure your compiler and
   binutils are up to date and see for yourself if LTO builds work for you.
   
   References:
   - https://gcc.gnu.org/wiki/LinkTimeOptimizationFAQ
   - #3481 (comment)
   - #3501
   - #3441
   
   (and a ton of other LTO tickets if you search for them)

   

   daniel-j-h committed Jan 5, 2017
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