Copy the Python runtime next to the executable only when the Python install has changed

CMake_modules/CopyPythonRuntime.cmake

@@ -0,0 +1,39 @@
+# Copies the Python runtime (Lib and DLLs) used by the embedded interpreter next to the executable.
+# Run as a post-build step: cmake -DPY_LIBDIR=... -DPY_DLLSDIR=... -DDEST_DIR=... -P CopyPythonRuntime.cmake
+#
+# Lib also carries the site-packages of whichever Python install the build found, which can be tens of thousands
+# of files. Copying them after every link took minutes, so the copy is skipped while the source install looks
+# unchanged since the last copy. Delete <exe dir>/python64/.eu07_python_runtime to force a refresh.
+
+foreach(_var PY_LIBDIR PY_DLLSDIR DEST_DIR)
+	if(NOT DEFINED ${_var})
+		message(FATAL_ERROR "CopyPythonRuntime.cmake: ${_var} is not set")
+	endif()
+endforeach()
+
+# directory timestamps change when pip adds or removes a package, or the Python install itself is replaced
+set(_stamp "")
+foreach(_path "${PY_LIBDIR}" "${PY_LIBDIR}/site-packages" "${PY_DLLSDIR}")
+	file(TIMESTAMP "${_path}" _time "%Y-%m-%dT%H:%M:%S" UTC)
+	string(APPEND _stamp "${_path} ${_time}\n")
+endforeach()
+
+set(_stamp_file "${DEST_DIR}/python64/.eu07_python_runtime")
+set(_old_stamp "")
+if(EXISTS "${_stamp_file}")
+	file(READ "${_stamp_file}" _old_stamp)
+endif()
+
+if(NOT _old_stamp STREQUAL _stamp)
+	message(STATUS "Copying the Python runtime to ${DEST_DIR}/python64")
+	foreach(_pair "${PY_LIBDIR}|Lib" "${PY_DLLSDIR}|DLLs")
+		string(REPLACE "|" ";" _pair "${_pair}")
+		list(GET _pair 0 _source)
+		list(GET _pair 1 _target)
+		execute_process(COMMAND "${CMAKE_COMMAND}" -E copy_directory_if_different "${_source}" "${DEST_DIR}/python64/${_target}" RESULT_VARIABLE _result)
+		if(NOT _result EQUAL 0)
+			message(FATAL_ERROR "CopyPythonRuntime.cmake: copying ${_source} failed")
+		endif()
+	endforeach()
+	file(WRITE "${_stamp_file}" "${_stamp}")
+endif()