bench: dynamically determine double-precision size

Author: kgryte

lib/node_modules/@stdlib/blas/base/zcopy/benchmark/fortran/benchmark.length.f

@@ -125,7 +125,7 @@ double precision function benchmark( iterations, len )
     ! External functions:
     interface
       subroutine zcopy( N, zx, strideX, zy, strideY )
-        complex(kind=8) :: zx(*), zy(*)
+        complex(kind=kind(0.0d0)) :: zx(*), zy(*)
         integer :: strideX, strideY, N
       end subroutine zcopy
     end interface
@@ -214,4 +214,4 @@ subroutine main()
     end do
     call print_summary( count, count )
   end subroutine main
-end program bench
+end program bench