[REG] -platform linux-clang-libc++ -c++std c++2b fails to compile synctqt

• *GIVEN*: a configuration which combines -platform linux-clang-libc++ and -c++std c++2b
• *WHEN*: trying to build
• *THEN*: the build fails with

    Target "syncqt" requires the language dialect "CXX23" (with compiler
    extensions).  But the current compiler "GNU" does not support this, or
    CMake does not know the flags to enable it.
  

It seems syncqt is always built with GNU-C++, even with -platform linux-clang, but the C++-standard is taken from -c++std.

Expected:

• either syncqt is build with the host compiler as requested by -platform, or
• CXX_STANDARD is hard-coded to what syncqt actually needs, not what was passed as -c++std.

Log file (with GCC and Clang versions, as well as config.opt) attached.