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

[ROCm/rocshmem commit: a1269e3db5]

projects/rocshmem/src/context.hpp

@@ -471,6 +471,11 @@ class Context {
    */
   int my_pe{-1};
 
+  /**
+   * @brief Duplicated local copy of backend's type
+   */
+  BackendType btype;
+  
   /**
    * @brief Stats common to all types of device contexts.
    */