While at it, removed support for Fedora 39 as it's EOL.

In 4.20, rpm changed the way build roots are named.  Our packaging
relies on the symlinks being created as a part of a rule to make sure
targets are being built correctly.

This patch implements (a hacky) way to check what the build root should
be named on a currently running rpm version and adjusts the symlinks
accordingly.

Wasm module is now not built for Amazon Linux 2, Debian 11 and Ubuntu
2.0.04, since it requires cmake version newer than what's available on
those OSes.  wasm-wasi-component is not affected.

Closes: https://github.com/nginx/unit/issues/915
Closes: https://github.com/nginx/unit/issues/1178

The 'Group' and 'BuildRoot' tags have been declared as deprecated by the
Fedora project.

Also, to quote the Fedora Packaging Guidelines

  The contents of the buildroot SHOULD NOT be removed in the first line
  of %install.

  The %defattr directive in the %files list SHOULD ONLY be used when
  setting a non-default value, or to reset to the default value after
  having set a non-default value.

Link: <https://fedoraproject.org/wiki/RPMGroups>
Link: <https://fedoraproject.org/wiki/Phase_out_buildroot_tag_(draft)>
Link: <https://fedoraproject.org/wiki/Archive:PackagingDrafts/BuildRoot>
Link: <https://docs.fedoraproject.org/en-US/packaging-guidelines/#_tags_and_sections>
[ Tweaked subject & added commit message - Andrew ]
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

Forgotten in bf3d5759e and 260494626.