[Bug lto/94259] --without-zstd seems to have no effect and links libzstd if available

[slyfox at inbox dot ru]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94259

--- Comment #1 from Sergei Trofimovich <slyfox at inbox dot ru> ---
I also noticed a minor infelicity: if you pass just --with-zstd build system
will  do a few unexpected things:
- it will not fail of zstd is not present in system but will silently skip zstd
support
- it will use 'yes/include' and 'yes/lib' as paths to zstd. I think it's never
an expected location.


1334 case "x$with_zstd" in
1335   x) ;;
1336   xno)
1337     ZSTD_INCLUDE=no
1338     ZSTD_LIB=no
1339     ;;
1340   *) ZSTD_INCLUDE=$with_zstd/include
1341      ZSTD_LIB=$with_zstd/lib
1342      ;;
1343 esac