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
I didn't like to use extern(C) aswell. But using externDFunc makes it "complicated" to use everywhere.
Could we add it as an import somewhere? Can we use externDFunc to define a function that is declared elsewhere?
I could also add another module core.config that contains drtConfigOptions and the extern(C) declarations of the drt_variables.