Use the Windows functions to get TCL functions on Cygwin.

This is related to linking semantics, so Cygwin should follow the Windows codepath.

setup.py

@@ -887,7 +887,7 @@ class pil_build_ext(build_ext):
         else:
             self._remove_extension("PIL._webp")
 
-        tk_libs = ["psapi"] if sys.platform == "win32" else []
+        tk_libs = ["psapi"] if (sys.platform == "win32" or sys.platform == "cygwin") else []
         self._update_extension("PIL._imagingtk", tk_libs)
 
         build_ext.build_extensions(self)

src/Tk/tkImaging.c

@@ -219,7 +219,7 @@ TkImaging_Init(Tcl_Interp *interp) {
 
 #define TKINTER_FINDER "PIL._tkinter_finder"
 
-#if defined(_WIN32) || defined(__WIN32__) || defined(WIN32)
+#if defined(_WIN32) || defined(__WIN32__) || defined(WIN32) || defined(__CYGWIN__)
 
 /*
  * On Windows, we can't load the tkinter module to get the Tcl or Tk symbols,

src/libImaging/ImPlatform.h

@@ -31,11 +31,18 @@
 #endif
 #endif
 
-#ifdef _WIN32
+#if defined(_WIN32) || defined(__CYGWIN__)
 
 #define WIN32_LEAN_AND_MEAN
 #include <Windows.h>
 
+#ifdef __CYGWIN__
+#undef _WIN64
+#undef _WIN32
+#undef __WIN32__
+#undef WIN32
+#endif
+
 #else
 /* For System that are not Windows, we'll need to define these. */