Allow link-time and run-time GC configuration

followup of #817:
- updated comments
- allow disabling command line processing through variable override at link time 
- allow enabling environment processing through variable override at link time 
- allow adding configuration option at link time 
- change environment var to DRT_GCOPT
- change default of maxPoolSize to 64
How about using extern(D) for this, there is a helper [externDFunc](https://github.com/D-Programming-Language/druntime/blob/master/src/core/internal/traits.d#L57) to declare external extern(D) functions.