You asked for a specific compression library, be clear when it isn't
found.

Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

This allows to actually build unit with support for zlib, zstd and
brotli compression.

Any or all can be specified. E.g.

  $ ./configure --zlib ...

  $ ./configure --zlib --zstd --brotli ...

During configure you will see if support for the requested compressions
has been found and what version of the library is being used.

E.g.

  ...
  checking for zlib ... found
   + zlib version: 1.3.1.zlib-ng
  checking for zstd ... found
   + zstd version: 1.5.6
  checking for brotli ... found
   + brotli version: 1.1.0
  ...
  Unit configuration summary:
  ...
    zlib support: .............. YES
    zstd support: .............. YES
    brotli support: ............ YES
  ...

Co-authored-by: Alejandro Colomar <alx@kernel.org>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>