nxcomp/src/Log.cpp: don't confuse users with useless warning messages in case that a log line append operation was requested, but the message level has lower precedence than the current log filter.

Also handle errors due to missing buffers gracefully.

This error is surprisingly easy to trigger, so we should make sure that
the program does not crash.

Fixes: ArcticaProject/nx-libs#596

The default function we use (if a thread name is not specified
explicitly) is pthread_self(). This function returns a number that is
guaranteed to be unique for each thread within a process, but this
assertion doesn't hold globally. Hence only using the thread ID is
ambiguous when logging from multiple processes.

This has one drawback: after flushing log data to its underlying output,
a new NXLogStamp object MUST be written to the NXLog object in order to
create a new entry within the stack.

This can be changed if necessary. For now I'd like to keep it as-is.

nxcomp/{configure.ac,Log.cpp}: implement configure-time std::put_time check and use macro value in Log.cpp.

Features:
	* Works without ifdefs
	* Configurable with commandline arguments
	* Log level configurable per file
	* Thread safe