add --with-idxtype configure option

Currently, we define the yac_int type based on the yaxt xt_int one. However, this dependency is not really required. It would be nice to the --with-idxtype in the YAC configure as well. It should define the following compiler macros:

• YAC_INT (C type for yac_int, e.g. int (default), long, or long long)
• YAC_FC_INT (Fortran integer kind, e.g. 1, 2, 4, or 8)
• YAC_INT_FMT (format string for printing yac_int using C stdout routines, e.g. "d")
• YAC_INT_MPIDT (MPI Datatype for yac_int, e.g. MPI_INT)
• XT_INT_MAX, XT_INT_MIN (minimum and maximum values of yac_int, e.g. INT_MIN/MAX)

Maybe this functionality could be copied from yaxt (if Thomas does not mind).

related #157 (closed)