Move windows header macros into orconfig.h

This should prevent duplicated code, and lower the likelihood of
accidentally making them inconsistent.

changes/win-macros

@@ -0,0 +1,7 @@
+  o Code simplification and refactoring:
+    - Define WINVER and _WIN32_WINNT centrally, in orconfig.h, in order
+      to ensure they remain consistent and visible everywhere.
+
+    - Remove vestigial workarounds for some ancient versions of the MSVC
+      compiler.
+