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
extern(C) is redundant here as you're overriding the mangling anyhow.