Set the default locale at startup.

On startup, the default locale is "C", meaning that all system
functions assume an ascii codeset. The environment's locale
settings should be selected by calling setlocale(LC_ALL,"").
This is done by Qt during the QCoreApplication initialization
but this inizialization is never performed for batch processing
and, as a result, LyX is not able to process files whose names
contain non-ascii characters. This is not an issue on Windows,
where the file names are always stored as UTF-16, so the call is
only performed for unix-like platforms (this also includes cygwin,
due to its own filenames management that allows using characters
which are forbidden to native programs).
Make binary file detection more robust.

The magic library can detect the charset used by a file. While this
detection is not full proof, actually the library seems to be infallible
as regards the binary nature of a file. So, use libmagic for the detection
and fallback to the previous method if the library is not installed or
its database cannot be loaded.
Cmake build: Added magic library to some link commands.

Needed due to the use of magic_(open|load|error|file|close) after recent changes.
Cmake build: Use LYX_USE_QT to determine if QT uses X11