RFR: Add guide section about platform-dependent nature of jextract [v3]

[Jorn Vernee]

On Wed, 17 Apr 2024 21:04:52 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> Jorn Vernee has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Add example of portable library practice
>
> doc/GUIDE.md line 102:
> 
>> 100: 
>> 101: However, it is also possible for a C library to be written in such a way that it is not
>> 102: platform dependent: a so-called _portable_ library. Sharing the bindings generated for a
> 
> Perhaps here we might say what moves portable libraries might do - such as use `long long` instead of `long`, or using sized numeric types e.g. `int32`.

Added this: 

> These libraries, for instance, use data types that have the same format on all supported platforms (such as `long long` instead of `long`, or an explicitly-sized integer type such as `int64_t`).

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PR Review Comment: https://git.openjdk.org/jextract/pull/237#discussion_r1570691904