Crash on Clang-cl compiler with MSVC linker in Windows

While compiling with Clang-cl, "__clang__" is defined by compiler internally, but "__attribute((weak))" is not supported by MSVC linker. This cause linker's interal error in alloc.c.

Start in alloc.c and reproduce the Windows build with Clang-cl using the MSVC linker. Inspect the handling of the __clang__ and weak-attribute conditions; done means the project compiles and links successfully with this toolchain without the linker internal error.

Written by the indexing model from the issue text.