jextract build fails on windows

[Daniel Jarabek]

Hi,

After the changes in [1], the jextractapp gradle task fails on Windows 
with the error `Error: no value given for --add-options`. While I'm not 
exactly sure what is happening here, wrapping the escaped quotes at [2] 
in another level of escaping (so they look like `\\\"`) resolves the 
issue. After this fix, the task still works successfully on linux (and 
presumably mac). I would make a pull request to fix this but I am unsure 
if this is the best (or even a valid) solution and due to my OCA 
currently being under review, the merging would be blocked anyways.

-DJ

[1] - https://github.com/openjdk/jextract/pull/11
[2] - 
https://github.com/openjdk/jextract/blob/e184943f15e8df68c6122b8f7e49c0a910a3b773/build.gradle#L113