[PATCH] meson: update doc logic for Windows

[Dmitry Kozlyuk]

2022-03-28 12:29 (UTC+0000), Varghese, Vipin:
[...]
> > We did get error from `doc/meson` stating ` echo command not available 
> > on windows` for both cmd and powershell for the line ``` 
> > run_target('doc', command: [echo, message, doc_target_names],
> >     depends: doc_targets)
> > ```  
> 
> This line never searches for "echo" command, e8c90926bdde ("doc: fix build on Windows with Meson 0.58") introduced "echo" variable precisely to avoid this.
> Can you give the full log?
> 
> > > Steps to reproduce the error:
> > >  - Install dependencies doxygen & sphinix build on Windwos server 2019.
> > >  - Build DPDK with option enable_docs=true for API or User Guide.
> > >
> > > This produces error
> > > ```
> > > FAILED: doc/api/examples.dox
> > > sh -e dpdk/doc/api/generate_examples.sh dpdk/examples 
> > > doc/api/examples.dox ```

Thanks for confirming that the "echo" issue did not reappear.

[...]
> Are you recommending 
> 
> Windows libraries and binaries should be cross build from linux (where all dependencies and shell script works)?
> 
> Or 
> 
> let us enhance the check for windows native build, if yes for option `-Denable_docs=true, we can stop the build for docs`.
> 
> If the latter is suggestion I will share v2 patch to check the same.

I recommend enhancing the Windows native build
by enabling full-fledged docs generation :)
I'm reluctant to accept this patch, even corrected,
because it effectively works against better Windows support:
- it makes less meson code reachable on Windows,
  allowing it to accumulate incompatible changes;
- it treats Windows as special,
  while we want it to be a platform supported on par with the others.
That being said, the latter option is acceptable for sure,
because it fixes a build failure in certain configuration.