Fix build with older Clang

[ZhongRuoyu]

This fixes the following build error:

  /tmp/micromamba-20230322-77117-1jorcxo/mamba-micromamba-1.4.0/libmamba/src/solv-cpp/queue.cpp:163:35: error: chosen constructor is explicit in copy-initialization
                  std::stringstream ss = {};
                                    ^    ~~
  /Library/Developer/CommandLineTools/SDKs/MacOSX11.sdk/usr/include/c++/v1/sstream:884:14: note: explicit constructor declared here
      explicit basic_stringstream(ios_base::openmode __wch = ios_base::in | ios_base::out);
               ^

The error was seen on macOS 11 (on both x86_64 and arm64) while packaging mamba for Homebrew at Homebrew/homebrew-core#126371. Build logs available here.

I am not sure about the root cause of this error, but it may be due to the lack of an implicit default constructor for std::stringstream in macOS 11 SDK's libc++. If I understand it correctly, this is related to the discussion on copy-list-initialization in https://stackoverflow.com/a/38419922.

[ZhongRuoyu]

Thanks @jonashaag! By the way, mamba 1.4.0 is not marked as the latest release on GitHub yet. Can we expect it to be announced soon?

[JohanMabille]

@ZhongRuoyu This is now fixed :)

[ZhongRuoyu]

Thanks, @JohanMabille!