Add build flags to control the region selector default

This will be used to run automated builds testing the new
region selectors.

hphp/runtime/base/runtime_option.cpp

@@ -398,6 +398,19 @@ static inline bool evalJitDefault() {
   return stat(path, &dummy) == 0;
 }
 
+static inline std::string regionSelectorDefault() {
+#ifdef HHVM_REGION_SELECTOR_TRACELET
+  return "tracelet";
+#else
+#ifdef HHVM_REGION_SELECTOR_LEGACY
+  return "legacy";
+#else
+  return "";
+#endif
+#endif
+}
+
+
 const uint64_t kEvalVMStackElmsDefault =
 #ifdef VALGRIND
  0x800

hphp/runtime/base/runtime_option.h

@@ -444,7 +444,7 @@ public:
   F(bool, HHIRDeadCodeElim,            true)                            \
   F(bool, HHIRPredictionOpts,          true)                            \
   F(bool, HHIRStressCodegenBlocks,     false)                           \
-  F(string, JitRegionSelector,         "")                              \
+  F(string, JitRegionSelector,         regionSelectorDefault())         \
   /* DumpBytecode =1 dumps user php, =2 dumps systemlib & user php */   \
   F(int32_t, DumpBytecode,             0)                               \
   F(bool, DumpTC,                      false)                           \