Fixes for Arch ARM

[pldemone]

Small fixes for Arch ARM and documenation changes

• Use tcflush() instead fsync() for data synchronization
• Set correct return type for getopt_long()
• "-lrt" shared linker flags added to CMakeLists.txt
• Short compile/cross-compile description added

[DocLambda]

The changes look fine except the "Use tcflush() instead fsync() for data synchronization" (f16a7b1) one. Please see my comment above.
Could you please prepare a pull request without f16a7b1!?

[pldemone]

Sure, but it will take some time because I'm out of office. Please note that fsync() is not working on many ARM platforms (probably RPi too) and the code will generate an error during execution. I my case it was intended to discard the queue to avoid a resend of invalid valid data and hitting a timeout. But I see that this solution was bogus for your platform.

[DocLambda]

Can you please give some pointers documenting that fsync is not working on ARM!? What is the exact error message? However, the best we can probably do is to test if the platform has a working fsync during configure (cmake) and ifdef out the calls to fsync otherwise...