Remove AC_C_CONST

Remove AC_C_CONST
It was breaking the build on Windows. The problem was that we included
stdio.h which gave a prototype for some functions (e.g. remove), then
the AC_C_CONST meant that we did
    /* Define to empty if `const' does not conform to ANSI C. */
    #define const /**/
and then we included io.h which gave prototypes that, due to const
being removed, conflicted with the earlier prototypes.