[CMake] If apinotes/ is a Git repository, re-run CMake when HEAD changes.

CMake doesn't have a nice, blessed way to do this (until 3.0), but most of
the time this will be good enough.

Swift SVN r21079

Author: jrose-apple

CMakeLists.txt

@@ -1600,6 +1600,14 @@ foreach(file ${SWIFT_API_NOTES_INPUT_FILES})
   list(APPEND SWIFT_API_NOTES_INPUTS ${name})
 endforeach()
 
+if(EXISTS "${SWIFT_API_NOTES_PATH}/.git/HEAD")
+  # Trigger updates if the apinotes repository version changes.
+  configure_file(
+    "${SWIFT_API_NOTES_PATH}/.git/HEAD"
+    "${CMAKE_BINARY_DIR}/apinotes-version.check_cache"
+    @ONLY)
+endif()
+
 # Add all of the subdirectories, where we actually do work.
 if (SWIFT_BUILD_TOOLS)
   add_subdirectory(include)