Parallelize optimization and code generation for link time optimization.
Parallelize optimization and code generation for link time optimization.

This parallelizes the `-flto` feature resulting in parallel optimization
and code generation for link time optimization based on the number of
logical processors available.

Note: this has the side-effect of using more memory during linking.

References:

- https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html (see: -flto)
- http://www.cmake.org/cmake/help/v3.0/module/ProcessorCount.htmMC

deniskoronchik/osrm-backend,yuryleb/osrm-backend,duizendnegen/osrm-backend,oxidase/osrm-backend,KnockSoftware/osrm-backend,deniskoronchik/osrm-backend,frodrigo/osrm-backend,KnockSoftware/osrm-backend,arnekaiser/osrm-backend,felixguendling/osrm-backend,felixguendling/osrm-backend,arnekaiser/osrm-backend,hydrays/osrm-backend,beemogmbh/osrm-backend,yuryleb/osrm-backend,raymond0/osrm-backend,hydrays/osrm-backend,Project-OSRM/osrm-backend,Project-OSRM/osrm-backend,beemogmbh/osrm-backend,frodrigo/osrm-backend,frodrigo/osrm-backend,hydrays/osrm-backend,KnockSoftware/osrm-backend,deniskoronchik/osrm-backend,bjtaylor1/osrm-backend,arnekaiser/osrm-backend,KnockSoftware/osrm-backend,deniskoronchik/osrm-backend,oxidase/osrm-backend,duizendnegen/osrm-backend,hydrays/osrm-backend,yuryleb/osrm-backend,neilbu/osrm-backend,raymond0/osrm-backend,bjtaylor1/osrm-backend,raymond0/osrm-backend,frodrigo/osrm-backend,duizendnegen/osrm-backend,bjtaylor1/osrm-backend,bjtaylor1/osrm-backend,beemogmbh/osrm-backend,oxidase/osrm-backend,Project-OSRM/osrm-backend,beemogmbh/osrm-backend,oxidase/osrm-backend,raymond0/osrm-backend,duizendnegen/osrm-backend,neilbu/osrm-backend,neilbu/osrm-backend,yuryleb/osrm-backend,Project-OSRM/osrm-backend,neilbu/osrm-backend,arnekaiser/osrm-backend,felixguendling/osrm-backend