Currently when Emacs is configured using the 
--enable-link-time-optimization configure flag, the build will use the 
-ffat-lto-objects flag if the host compiler is GCC (but not Clang). 
According to the comments in configure.ac this is to work around an 
issue with LTO builds in GCC 4.9.0. This flag makes builds take much 
longer (and produces much larger object files), so I suggest removing it 
as GCC 4.x is quite old at this point and doesn't have great LTO support 
anyway.

Patch to remove this logic from configure.ac is attached inline below 
(generated using git format-patch). I agree to copyright assignment of 
this change to the FSF.