This module applies specifically to the "double-double" long double arrangement - such as
provided by the PowerPC processors.

Install in the usual way:

 perl Makefile.PL
 make test
 make install

If your perl's NV (float) is not such a 'double-double', the Makefile.PL should detect
this and refuse to create a makefile - and you won't be able to build and install this
module using that perl.
That's the way it should work, anyway. Please file a bug report if you strike deviant
behaviour.
