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 want to merge sub-options from the different sources, e.g. precise is set by rt_options, but you want to tweak incPoolSize from the command line. rt_configOptions only works for single options.

The alternative would be to specify GC options as single options with a prefix, e.g. "gc_precise". Then rt_configOption could be called on every option. It could get quite verbose on the command line, though.