Do not force a specific c(++) compiler by setting the
CMAKE_{}_COMPILER variable.
After the c(++) compiler binary has been choosen check if it is the GNU compilear.

In this way we allow the users to specify a GNU compiler under a non
standard path through the CC and CXX envirnoment variable.

Preload lib: RPC/IPC Mercury/Margo environments are only initialized if the mountdir path is used by the application
For that, the daemon writes the mountdir (it was started with) to a configurable path to disk.
The libraries read this value and only the process that uses part of this path will actually trigger the environment initialization.
This forbids prior behavior where applications initialize the whole environment numerous times, although unused.
For example, any MPI program will spawn multiple processes, all with the LD_PRELOAD environment variable set.

Configure file is split into a public and private part to distinguish file system configurations from monitoring, for example

Added: register daemon to system for storing auxiliary files + better daemon startup and shutdown handling