make GC configurable through environment variable

Here is a proposal to get the ball rolling. It adds gc.config to configure profiling and pool limits, allowing to experiment with other limits within the GC.

To fix your application to other defaults or to avoid dependency on the environment variable D_GC, replace it with your own version of gc.config with the same struct layout and pass it explicitely to the linker.
> main.c? :)

Ooops ;-)

The comment has become slightly outdated, I'll follow up. Does this PR need to be reflected somewhere in the docs (where would that be?), or is an entry in the changelog good enough?