configure.ac: fix brotli/zstd configure argument name; fix zstd searching

commit 33a9c32a31b246a10af5aaf413ae6e927810e723
Author: Sam James <sam@gentoo.org>
Date:   Fri Mar 18 05:51:29 2022 +0000

    configure.ac: fix zstd detection

    Fixes typo in zstd detection.

    None of the used autoconf macros will define `has_zstd_h` so
    configure will (AFAICT) always fail to find zstd, even if it succeeded
    via pkg-config moments before.

    Drop it and rely solely on pkg-config as that's the only search
    we're actually doing.

    Fixes: https://gitlab.com/gnutls/gnutls/-/issues/1343
    Signed-off-by: Sam James <sam@gentoo.org>

commit 6b794e49d1a14e43f9e08023f958364712c3c89a
Author: Sam James <sam@gentoo.org>
Date:   Fri Mar 18 05:40:28 2022 +0000

    configure.ac: fix brotli/zstd configure argument name

    The old `./configure` arguments for brotli and zstd respectively
    were inconsistent with the `./configure --help` output.

    Old: --without-libbrotli --without-libzstd (also --with-*)
    New: --without-brotli --without-zstd (also --with-*)

    Fixes: https://gitlab.com/gnutls/gnutls/-/issues/1342
    Signed-off-by: Sam James <sam@gentoo.org>

Checklist

• Commits have Signed-off-by: with name/author being identical to the commit author
• Code modified for feature
• Documentation updated / NEWS entry present (for non-trivial changes)
• CI timeout is 2h or higher (see Settings/CICD/General pipelines/Timeout)

Reviewer's checklist:

• Any issues marked for closing are addressed
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTION.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code