windows/README: Use CROSS_COMPILE instead of CC to cover other tools too.

Suggested by @lurch.

windows/README

@@ -4,7 +4,7 @@ It is based on Unix port, and expected to remain so.
 To cross-compile under Debian/Ubuntu Linux system:
 
 sudo apt-get install mingw32 mingw32-binutils mingw32-runtime
-make CC=i586-mingw32msvc-gcc
+make CROSS_COMPILE=i586-mingw32msvc-
 
 The port requires additional testing, debugging, and patches. Please
 consider to contribute.