Users may use -DCMAKE_CXX_FLAGS_DEBUG to add addition compiler flags for debug mode.

-g -O2 has been replaced with -O3 in all cases.

Upgrate rocksDB deps

See merge request ada-fs/fs!5

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.

5.10.2 -> 5.10.3

5.10.2 couldn't be compiled with gcc 7 due to a rocksDB bug [1]

[1]: https://github.com/facebook/rocksdb/issues/3486

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.