Update CPM to 0.40.0 and add SPDX license headers

cmake/CPM.cmake

@@ -1,25 +1,24 @@
-set(CPM_DOWNLOAD_VERSION 0.35.0)
+# SPDX-License-Identifier: MIT
+#
+# SPDX-FileCopyrightText: Copyright (c) 2019-2023 Lars Melchior and contributors
+
+set(CPM_DOWNLOAD_VERSION 0.40.0)
+set(CPM_HASH_SUM "7b354f3a5976c4626c876850c93944e52c83ec59a159ae5de5be7983f0e17a2a")
 
 if(CPM_SOURCE_CACHE)
-  # Expand relative path. This is important if the provided path contains a
-  # tilde (~)
-  get_filename_component(CPM_SOURCE_CACHE ${CPM_SOURCE_CACHE} ABSOLUTE)
-  set(CPM_DOWNLOAD_LOCATION
-      "${CPM_SOURCE_CACHE}/cpm/CPM_${CPM_DOWNLOAD_VERSION}.cmake")
+  set(CPM_DOWNLOAD_LOCATION "${CPM_SOURCE_CACHE}/cpm/CPM_${CPM_DOWNLOAD_VERSION}.cmake")
 elseif(DEFINED ENV{CPM_SOURCE_CACHE})
-  set(CPM_DOWNLOAD_LOCATION
-      "$ENV{CPM_SOURCE_CACHE}/cpm/CPM_${CPM_DOWNLOAD_VERSION}.cmake")
+  set(CPM_DOWNLOAD_LOCATION "$ENV{CPM_SOURCE_CACHE}/cpm/CPM_${CPM_DOWNLOAD_VERSION}.cmake")
 else()
-  set(CPM_DOWNLOAD_LOCATION
-      "${CMAKE_BINARY_DIR}/cmake/CPM_${CPM_DOWNLOAD_VERSION}.cmake")
+  set(CPM_DOWNLOAD_LOCATION "${CMAKE_BINARY_DIR}/cmake/CPM_${CPM_DOWNLOAD_VERSION}.cmake")
 endif()
 
-if(NOT (EXISTS ${CPM_DOWNLOAD_LOCATION}))
-  message(STATUS "Downloading CPM.cmake to ${CPM_DOWNLOAD_LOCATION}")
-  file(
-    DOWNLOAD
-    https://github.com/cpm-cmake/CPM.cmake/releases/download/v${CPM_DOWNLOAD_VERSION}/CPM.cmake
-    ${CPM_DOWNLOAD_LOCATION})
-endif()
+# Expand relative path. This is important if the provided path contains a tilde (~)
+get_filename_component(CPM_DOWNLOAD_LOCATION ${CPM_DOWNLOAD_LOCATION} ABSOLUTE)
+
+file(DOWNLOAD
+     https://github.com/cpm-cmake/CPM.cmake/releases/download/v${CPM_DOWNLOAD_VERSION}/CPM.cmake
+     ${CPM_DOWNLOAD_LOCATION} EXPECTED_HASH SHA256=${CPM_HASH_SUM}
+)
 
 include(${CPM_DOWNLOAD_LOCATION})