Use isfinite function for ARM compatibility

[alexreg]

The finite function doesn't exist on ARM architectures.

admutils.c:459:9: error: call to undeclared library function 'finite' with type 'int (double)'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
   if (!finite(a[i])) fatalx("(testnan) fails:  index %d\n",i) ;
        ^
admutils.c:459:9: note: include the header <math.h> or explicitly provide a declaration for 'finite'
1 error generated.
make: *** [admutils.o] Error 1

[alexreg]

A new release following this would be great too, if you wouldn't mind.