gc.config: updated comments, allow disabling command line processing, change environment var to DRT_GCOPT
Merge branch 'master' of https://github.com/D-Programming-Language/druntime into gc_config_cleanup
configure options at link time
move drt_ variables to rt.config
add function drtConfigOption to retreive an option
show current settings in GC help message
use rt_ prefix instead of drt_, remove extern(C)
add C mangling depending on toolchain (needed for OSX)
- pass optional delegate to rt_configOption to allow processing each found option
- use rt_configOption from gc.config
Merge pull request #986 from rainers/gc_config_cleanup

Allow link-time and run-time GC configuration