A macro to check the options of nice, in order to have a
VERY_NICE variable which runs a program at the lowest priority
VERY_NICE is undefined if we don't find the proper options, so
you can safely use:
in a shell script.
The VERY_NICE variable includes the placeholder NICE_VALUE that
you have to instantiate at run-time.
If you give a argument to AC_VERY_NICE, it will be used as an
argument of nice for testing and included in VERY_NICE instead of
the above placeholder.