Call xlib functions using pointers
Implement dynamic loading of the libX11.so

Controlled by the --with-dyn-X11 flag of the configure script.
Document --with-dyn-X11 flag of configure script

It controls static/dynamic linking against libX11 library.

Thanks to Hendrik Jaeger (a.k.a. henk) for the idea.
Display --with-dyn-X11 flag status in version info
Use format_str() function in version.c unit
Merge branch 'dynamic-xlib-loading'