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_host.cpp

@@ -31,7 +31,8 @@ namespace rocshmem {
 __host__ Context::Context(Backend* handle, bool shareable)
     : num_pes(handle->getNumPEs()),
       my_pe(handle->getMyPE()),
-      fence_(shareable) {
+      fence_(shareable),
+      btype(handle->type) {
 }
 
 __host__ Context::~Context() {