make check fails on arm-linux

[Clint Adams]

> True, but that's just a band-aid.  It'll hurt us later on.  Given my 
> druthers, let's identify the problem and do we need in order to fix it.  If 
> it's a rounding problem, or a narrow double (32 bits instead of 64 bits), 
> or some such thing that we can't fix with our code, then maybe we should 
> skip this test for that architecture.  However, that's a last resort.

I'll add AC_CHECK_SIZEOF(double) to configure.in to try to eliminate
that concern.