Header path handling on Windows
This issue is possibly related to #284, but it seemed to me like a distinct issue and I was still poking at the problem, so I waited to raise it.

To be more precise, I ran into this problem *after* I put a workaround in place for #284.

I'm using nimterop to work with a library that provides alternative header files, the choice of one or another header depending on which lib features you plan to use.

So I have this kind of thing in my wrapper:
```nim
const thelibHeaderRelPath {.strdefine.} =
  "include" / "thelib" / "some.h"
```
And I pass `thelibHeaderRelPath` as the first argument to nimterop's `getHeader`, along with other appropriate arguments.

On Linux and macOS, this approach works as I expected. *But* it does *not* work on Windows, where my build environment is an MSYS2 Bash shell in MSYS2's UCRT64 environment.

After poking around in nimterop, I hit on a solution that involves a change in *my* code instead of nimterop's:
```nim
const thelibHeaderRelPath {.strdefine.} =
  when defined(windows):
    "include\\\\thelib\\\\some.h"
  else:
    "include" / "thelib" / "some.h"
```
Given that it works, it's suggestive that somewhere in nimterop's `getHeader` macro, and/or a helper function, path handling is having an issue with the `\` path separator on Windows (or maybe just Windows when the Nim compiler is invoked from MSYS2 Bash, I'm not sure).

I've tried to figure out a change/fix to the nimterop code, but have so far been unsuccessful.