Re: make installcheck-world in a clean environment

To avoid overheating of this pretty hot discussion, I would like just to propose "a more elaborated fix" (for REL_10_STABLE and master).
In fact, when we perform "make installcheck" it not only requires us to build ecpg, but it also rebuilds libpostgres, libpgport and libpq (for installcheck-world).
I believe that the larger testing surface (coverage), the better, so using installed assets (libs, headers) is more useful.