Use MMTK Option env var support.

This will enable us to configure a lot of the MMTK specific settings
(including the plan, the number of threads, and GC stress controls)
using MMTK_ prefixed env vars

gc/mmtk/src/api.rs

@@ -12,7 +12,6 @@ use mmtk::memory_manager;
 use mmtk::memory_manager::mmtk_init;
 use mmtk::util::constants::MIN_OBJECT_SIZE;
 use mmtk::util::options::GCTriggerSelector;
-use mmtk::util::options::PlanSelector;
 use mmtk::util::Address;
 use mmtk::util::ObjectReference;
 use mmtk::util::VMMutatorThread;
@@ -37,13 +36,9 @@ pub extern "C" fn mmtk_is_reachable(object: ObjectReference) -> bool {
 
 #[no_mangle]
 pub extern "C" fn mmtk_builder_default() -> *mut MMTKBuilder {
-    let mut builder = MMTKBuilder::new_no_env_vars();
+    let mut builder = MMTKBuilder::new();
     builder.options.no_finalizer.set(true);
 
-    // Hard code NoGC for now
-    let plan_selector = "MarkSweep".parse::<PlanSelector>().unwrap();
-    builder.options.plan.set(plan_selector);
-
     // 100MiB
     builder.options.gc_trigger.set(GCTriggerSelector::FixedHeapSize(100 << 20));