cannot use log4cxx header files with clang++

I initially reported the problem for vcpkg log4cxx package:

https://github.com/microsoft/vcpkg/issues/19730

Here's the report text:

—

[log4cxx] failure to compile application with clang on Windows/VC2019

We have an application that uses log4cxx.
When I compile with clang on Windows VC2019 I observe the following error:
[...]
In file included from C:\vcpkg\installed\x64-windows\include\log4cxx/logger.h:30:
In file included from C:\vcpkg\installed\x64-windows\include\log4cxx/helpers/appenderattachableimpl.h:27:
In file included from C:\vcpkg\installed\x64-windows\include\log4cxx/spi/appenderattachable.h:30:
C:\vcpkg\installed\x64-windows\include\log4cxx\appender.h(140,1): error : explicit instantiation of 'allocator' not in a namespace enclosing 'std'
LOG4CXX_LIST_DEF(AppenderList, AppenderPtr);
^
C:\vcpkg\installed\x64-windows\include\log4cxx\log4cxx.h(91,45): note: expanded from macro 'LOG4CXX_LIST_DEF'
extern template class LOG4CXX_EXPORT std::allocator;
^
C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional\VC\Tools\MSVC\14.27.29110\include\iosfwd(167,7): note: explicit instantiation refers here
class allocator;
^

I found this error appears because clang compiler included with Visual Studio has _MSC_VER macro defined.
(Intel icx compiler also has _MSC_VER macro defined on Windows)

The simple fix worked for me for both compilers:
(log4cxx.h, line 81)

#elif defined(_MSC_VER) && !defined(__clang__) && !defined(LOG4CXX_STATIC)

I didn't try to compile with LOG4CXX_STATIC defined, though.

—

vcpkg folks recommended to address "upstream" team.