Convert the CPACK_CYGWIN_PATCH_NUMBER variable to a cache variable

Makes it easy to override it, by priming the cache from a release
build script, for example.

Also: correct mistaken @@ variable references with plain ${}
style references so that future readers do not think that it
must be an input to configure_file...

CMakeCPack.cmake

@@ -99,13 +99,14 @@ if(EXISTS "${CMAKE_ROOT}/Modules/CPack.cmake")
     set(CPACK_SOURCE_PACKAGE_FILE_NAME ${CPACK_PACKAGE_FILE_NAME})
     # Create a cygwin version number in case there are changes for cygwin
     # that are not reflected upstream in CMake
-    set(CPACK_CYGWIN_PATCH_NUMBER 1)
+    set(CPACK_CYGWIN_PATCH_NUMBER 1 CACHE STRING "patch number for CMake cygwin packages")
+    mark_as_advanced(CPACK_CYGWIN_PATCH_NUMBER)
     # These files are required by the cmCPackCygwinSourceGenerator and the files
     # put into the release tar files.
     set(CPACK_CYGWIN_BUILD_SCRIPT
-      "${CMake_BINARY_DIR}/@CPACK_PACKAGE_FILE_NAME@-@CPACK_CYGWIN_PATCH_NUMBER@.sh")
+      "${CMake_BINARY_DIR}/${CPACK_PACKAGE_FILE_NAME}-${CPACK_CYGWIN_PATCH_NUMBER}.sh")
     set(CPACK_CYGWIN_PATCH_FILE
-      "${CMake_BINARY_DIR}/@CPACK_PACKAGE_FILE_NAME@-@CPACK_CYGWIN_PATCH_NUMBER@.patch")
+      "${CMake_BINARY_DIR}/${CPACK_PACKAGE_FILE_NAME}-${CPACK_CYGWIN_PATCH_NUMBER}.patch")
     # include the sub directory cmake file for cygwin that
     # configures some files and adds some install targets
     # this file uses some of the package file name variables