Build on Windows

I'm trying to build this on Windows for Python 3.6, since I'd like to use Loopy in a project, where we ship packages to Mac, Linux & Win. I've started by trying to see if MSVC 14 could build ISL, but I'm becoming sceptical:

A few things I've done in `isl/include/isl/ctx.h`,
```c
#define __attribute__(...)
```
and in `isl/isl_arg.c:20` to
```c
static struct isl_arg *help_arg;
```

Finally, I'm seeing at the end of the build
```
isl_hash.obj : error LNK2001: unresolved external symbol ffs
isl_stream.obj : error LNK2001: unresolved external symbol strcasecmp
build\lib.win-amd64-3.6\islpy\_isl_cffi.pyd : fatal error LNK1120: 2 unresolved externals
```
It looks like `strcasecmp` is a [known one](http://botsikas.blogspot.fr/2011/12/strcasecmp-identifier-not-found-when.html) and `ffs` is a [GCC builtin](https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html), so those can likely be mapped to Windows equivalents.

I'm worried that ignoring `__attribute__` might hurt ISL correctness.

Another route would be to compile it all with GCC on MinGW 64, but I'm not sure how compatible this would be with the standard Python packages. Could that be used to produce wheels, etc?

_edited to increase SNR_