Changing isatty() declaration to unistd.h include

This change has been discussed with Andrew Martin over emails (with
Craig Porter cc'ed). It should have no effect on code that already
compiles correctly.

Motivation: the hard-coded declaration of isatty() slightly differs from
the official one, which conditionally adds a throw specification if
being compiled under a C++ compiler. This sometimes breaks the compile
when I include, say "pdb.h", from C++ with:

    extern "C" {
      #include "bioplib/trunk/pdb.h"
    }

...because the C++ compiler has already seen a definition of isatty()
with a throw specification elsewhere and barfs on seeing a conflicting
declaration.