cmake update needed

[Tony Kelman]

Thanks for your input and taking an interest in the specifics Corinna.
Coincidentally upstream just released 3.1.2, so I may as well build and
upload that version with the patches added back in.

> > Second, it corrects some /proc/cpuinfo, /proc/meminfo, etc 
> > special-casing
> > of Cygwin where using the same code as Linux should work. Makes sense,
> > I'll split this out so it can be discussed on its own w/upstream.
>
> Would be nice to learn the details. Maybe we can tweak this in Cygwin
> for the future...?

Looking closer at Source/kwsys/SystemInformation.cxx, now I'm not as sure.
Without the patch, the special-case behavior for Cygwin is looking for
"cpu count" in /proc/cpuinfo, which I don't see on my machine. The Linux
code that the patch switches to look for "physical id" and "cpu cores,"
which I also don't see.

The changes to read "MemTotal:" and "MemFree:" from /proc/meminfo,
"VmRSS:" from /proc/self/status, and having GetProcessId() use getpid()
instead of returning -1 should work fine though.

> Yes. It's not the task of the application to second-guess the
> underlying "OS" (Cygwin taking the role in a way).

There are various wrapper scripts in autotools to do the same thing
(use Cygwin as a build environment for non-Cygwin compilers), they just
tend to make decisions on what kinds of path translation to do based on
`uname` rather than compile-time #ifdef's. Though with CMake, users can
always have the option of using the binary installer downloads and
running those from within Cygwin. So the Cygwin-distributed build of
cmake has a bit more room to follow Cygwin best practices, at the cost
of not supporting as many use cases, and requiring local patches that
need to be kept updated.

> Cygwin's access() call is not for POSIX paths only.
> It handles native Windows paths as well.

Oh cool, I was not aware of that.

> As for FileIsFullPath, what is the change about?

The change disables the following snippet of code for the Cygwin case:

  // On Windows, the name must be at least two characters long.
  if(len < 2)
    {
    return false;
    }
  if(in_name[1] == ':')
    {
    return true;
    }
  if(in_name[0] == '\\')
    {
    return true;
    }

instead using the Unix condition of false if len < 1, true if
in_name[0] == '~' or '/', false otherwise. The (len < 2) part is
obviously wrong for Cygwin, but this function isn't using access() so
the change would make this function no longer return true for C:\ style
Windows paths or UNC paths. A Cygwin application should probably be
given those paths in /cygdrive/c/ form, but I'm not sure of all the
places in cmake's code where this function gets used and what the
consequences are.

-Tony