CMake: Add backport of PROJECT_IS_TOP_LEVEL for CMake <3.21

CMakeLists.txt

@@ -6,7 +6,7 @@
 # Set Project Properties
 ##################################
 
-cmake_minimum_required(VERSION 3.21.0)
+cmake_minimum_required(VERSION 3.12.0)
 
 #Project Name
 project(netCDF
@@ -16,6 +16,14 @@ DESCRIPTION "NetCDF is a set of software libraries and machine-independent data
 )
 set(PACKAGE "netCDF" CACHE STRING "")
 
+# Backport of built-in `PROJECT_IS_TOP_LEVEL` from CMake 3.21
+if (NOT DEFINED NETCDF_IS_TOP_LEVEL)
+  set(NETCDF_IS_TOP_LEVEL OFF)
+  if (CMAKE_CURRENT_SOURCE_DIR STREQUAL CMAKE_SOURCE_DIR)
+    set(NETCDF_IS_TOP_LEVEL ON)
+  endif ()
+endif ()
+
 #####
 # Version Info:
 #
@@ -2787,6 +2795,8 @@ install(
 ####
 
 # CPack inclusion must come last.
-if (PROJECT_IS_TOP_LEVEL)
+option(NETCDF_PACKAGE "Create netCDF-C package " ${NETCDF_IS_TOP_LEVEL})
+
+if (NETCDF_PACKAGE)
   include(CMakeInstallation.cmake)
 endif()