Add a runtime flag to enable MMTk #1
Description
In my initial iteration of the Ruby/MMTk binding, the Ruby compiler modifications are enabled/disabled via a compiler flag. Although this should be functional, it won't be ideal because the entire language must be rebuilt whenever we want to turn the third party heap on/off (e.g. to perform comparative benchmarks etc).
My initial attempt to implement a runtime flag was to add a feature as a command line option to the ruby interpreter. However, this attempt proved unfruitful because the VM/heap is initialised before the command line arguments are parsed, which makes it impossible to configure MMTk usage without majorly refactoring the code (something which I am not comfortable doing).
Chris's suggestion was to use an environment variable instead. The get_param function in vm.c may be useful for this.