bugfix: uninitialized position on new OpenFile. 

See merge request ada-fs/fs!7

better handling of mandatory cli parameters

See merge request ada-fs/fs!6

The file position of a newly created OpenFile
it wasn't set correctly to 0.

See https://xgitlab.cels.anl.gov/sds/margo/issues/40 for reference of the issue.

Through boost::program_option it is possible to specify which are
the mandatory parameter and to print a proper error message in case some
of them is missing.

Download script: force git checkout

See merge request ada-fs/fs!4

CMake gnucc fixes

See merge request ada-fs/fs!2

The download script can be used to update dependencies source code by
pull the new version and performing a checkout to that version.
Without forcing (-f) the checkout files that have been modified or
patched won't be updated correctly.

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.