Use gnu_printf in attribute(()) to indicate GNU printf extension support.

Use gnu_printf in attribute(()) to indicate GNU printf extension support.

By writing ordinary printf (or __printf__) inside __attribute__((format)),
one indicates that the system's typical format specifiers are supported.
This is OK for building for glibc, but it falls down for building for
mingw, since PSPP actually supports GNU format specifiers there via gnulib.
By writing gnu_printf instead, we avoid some incorrect warnings on mingw.