Add anti-YAGNI clarifying comment to src/win32_shim.h

Author: python-processing-unit

src/win32_shim.h

@@ -1,6 +1,11 @@
 #ifndef WIN32_SHIM_H
 #define WIN32_SHIM_H
 
+// DO NOT REMOVE: Clang on Windows with --driver-mode=cl does NOT provide
+// <threads.h>. MSVC does since VS 2015, but the UCRT shipped with Clang-cl
+// lacks a conforming C11 threads implementation. This file reimplements
+// <threads.h> using Win32 primitives and is required for all builds.
+
 #ifdef _WIN32
 
 #define WIN32_LEAN_AND_MEAN