HAVE_X11_XLIB_H is used in several files (unfortunately)

Ensure that stdio and libintl won't do heap allocations beforehand.

Refer to 7e5403d0 for rationale.

...unless the buffer has zero size.
This is not to be confused with strncpy().

Changing the environment is not thread-safe, so it's best avoided. As a
replacement, users should set the variable in their shell startup
configuration (e.g. /etc/environment or ~/.bash_profile). Then all
applications using DVDCSS will be affected (which seems better).
This option was probably very seldom used anyway.

Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>