move download and extract code into function

cmake/Modules/ExternalCMakeProject.cmake

@@ -0,0 +1,35 @@
+# Build a CMake based external library as subdirectory.
+# The sources will be unpacked to ${CMAKE_BINARY_DIR}/_deps/${target}-src
+# The binaries will be built in ${CMAKE_BINARY_DIR}/_deps/${target}-build
+#
+function(ExternalCMakeProject target url hash basedir cmakedir)
+  # change settings locally
+  set(BUILD_SHARED_LIBS OFF)
+  set(CMAKE_POSITION_INDEPENDENT_CODE ON)
+
+  if(CMAKE_VERSION VERSION_LESS 3.14)
+    get_filename_component(archive ${url} NAME)
+    file(MAKE_DIRECTORY ${CMAKE_BINARY_DIR}/_deps/src)
+    message(STATUS "Downloading ${url}")
+    file(DOWNLOAD ${url} ${CMAKE_BINARY_DIR}/_deps/${archive} EXPECTED_HASH MD5=${hash} SHOW_PROGRESS)
+    message(STATUS "Unpacking and configuring ${archive}")
+    execute_process(COMMAND ${CMAKE_COMMAND} -E tar xzf ${CMAKE_BINARY_DIR}/_deps/${archive}
+      WORKING_DIRECTORY ${CMAKE_BINARY_DIR}/_deps/src)
+    file(GLOB TARGET_SOURCE "${CMAKE_BINARY_DIR}/_deps/src/${basedir}*")
+    list(LENGTH TARGET_SOURCE _num)
+    if(_num GREATER 1)
+      message(FATAL_ERROR "Inconsistent ${target} library sources. "
+        "Please delete ${CMAKE_BINARY_DIR}/_deps/src and re-run cmake")
+    endif()
+    file(REMOVE_RECURSE ${CMAKE_BINARY_DIR}/_deps/${target}-src)
+    file(RENAME ${TARGET_SOURCE} ${CMAKE_BINARY_DIR}/_deps/${target}-src)
+    add_subdirectory("${CMAKE_BINARY_DIR}/_deps/${target}-src/${cmakedir}"
+      "${CMAKE_BINARY_DIR}/_deps/${target}-build")
+  else()
+    include(FetchContent)
+    message(STATUS "Downloading ${url}")
+    FetchContent_Declare(${target} URL ${url} URL_HASH MD5=${hash} SOURCE_SUBDIR ${cmakedir})
+    message(STATUS "Unpacking and configuring ${archive}")
+    FetchContent_MakeAvailable(${target})
+  endif()
+endfunction(ExternalCMakeProject)