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/util.cpp

@@ -26,6 +26,8 @@
 
 #include <cstdio>
 #include <cstdlib>
+#include <cctype>
+#include <algorithm>
 #include <vector>
 
 #include "rocshmem/rocshmem_config.h"  // NOLINT(build/include_subdir)