Add "prealloc" variable to configure script

sar (or sadc) allocates empty records in the data files it creates so
that it can save statistics for devices (disks, network interfaces,
etc.) that may be added to the system after the file was created.
The drawback is that data files take more space on disk than actually
strictly necessary.
Using the "prealloc" variable with configure (before compiling sysstat),
the user can tell how much space he wants to allocate.
This variable will determine the size of data files created by sar/sadc.
The default value is 1, meaning that some empty records will be
allocated.
A value of 0 means that data files will be the smallest possible.