Remove detection of X11 from configure.ac.
Add detection of SDL using sdl2-config.
Minor update to the README.
Replace Find_file with the Boost filesystem library.
Remove some unused variables from main.h.
Use the Boost filesystem library in a Win32 compatible way.
Signal handling is not needed on Win32.
Use the "main" signature specified by SDL.
Remove the checks for GNU allocation functions.

We do not need the GNU specific functionaliy, and on Win32, this breaks them.
Disable the clipboard handling on Win32.