Use FetchContent to enable standalone CMake testing

cmake/FetchContent/CMakeLists.cmake.in

@@ -0,0 +1,21 @@
+# Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+# file Copyright.txt or https://cmake.org/licensing for details.
+
+cmake_minimum_required(VERSION ${CMAKE_VERSION})
+
+# We name the project and the target for the ExternalProject_Add() call
+# to something that will highlight to the user what we are working on if
+# something goes wrong and an error message is produced.
+
+project(${contentName}-populate NONE)
+
+include(ExternalProject)
+ExternalProject_Add(${contentName}-populate
+                    ${ARG_EXTRA}
+                    SOURCE_DIR          "${ARG_SOURCE_DIR}"
+                    BINARY_DIR          "${ARG_BINARY_DIR}"
+                    CONFIGURE_COMMAND   ""
+                    BUILD_COMMAND       ""
+                    INSTALL_COMMAND     ""
+                    TEST_COMMAND        ""
+)