allow all three backends to co-exist in a single build (#270)

* add support for compiling all backends

also include the logic to select backends either based on user requests
or through some heuristics

* checkpoint for compiling all backends

* final checkpoint

all tests seem to pass when compiling all three backends simultaneasly
and forcing to use any of the three Backends.

* update PR to new envvar system

src/envvar.cpp

@@ -46,6 +46,7 @@ namespace envvar {
     const var<size_t> max_wavefront_buffers("MAX_WF_BUFFERS", "", 1024);
     const var<std::string> requested_dev("USE_IB_HCA", "");
     const var<uint32_t> sq_size("SQ_SIZE", "", 1024);
+    const var<std::string> backend("BACKEND", "");
   }  // inline namespace _base
 
   namespace bootstrap {