Merge files build

[Yuriy Skalko]

> There was no big advantage (in speed) anymore.
> 
> Anyway, if you take care of it, I don't object.
> 
>         Kornel

Yes, the only advantage should be speed of build, but it is now similar 
to normal build even on my 4-core laptop. Perhaps on modern 8-16-32-core 
machines it can be even slower.
Sure it is not worth to take care of it anymore.


> We used to have this in autoconf (monolithic build) and we decided to remove it in 2.4.0. Isn't link-time-optimization doing the same thing?
> 
> 
> JMarc

I think so. But adding "-flto" for my GCC 7.3 breaks compilation. On 
newer versions it should be better.

Yuriy