RE: Improve docs syntax checking and enable it in the meson build

Dear Nazir,

> I see your point but the suggested change actually doesn't affect C
> files' compilation. They are still built although you make this
> change, AFAIK this is how meson build works.

Did you say that C files would be anyway built even after applying my change?
I don't think so - I have verified that only needed documents were built with
the `meson test --suite doc` command after I modified meson.build.

```
$ ninja clean
[2/2] Cleaning
Cleaning... 8 files.
$ meson test --suite doc
ninja: Entering directory `/home/hayato/builddir'
[8/8] Generating doc/src/sgml/postgres-full.xml with a custom command // only 8 files are generated
1/1 postgresql:doc / doc/sgml_syntax_check OK 0.38s

Ok: 1
Expected Fail: 0
Fail: 0
Unexpected Pass: 0
Skipped: 0
Timeout: 0

Full log written to /home/hayato/builddir/meson-logs/testlog.txt
```

> Meson has '--no-rebuild' option for these types of situations, could
> you try running "meson test --suite doc --no-rebuild" and see if that
> helps?

Confirmed that `--no-rebuild` option can avoid building C files, but I still
think it should be handled on the meson.build side.

Best regards,
Hayato Kuroda
FUJITSU LIMITED