Polyfill fetchcontent_makeAvailable for cmake < 3.14

CMakeLists.txt

@@ -46,6 +46,32 @@ option(USE_BUNDLED_TWEENY "Use the bundled version of tweeny."
 
 list(APPEND CMAKE_MODULE_PATH "${CMAKE_SOURCE_DIR}/cmake")
 
+if(${CMAKE_VERSION} VERSION_LESS "3.14.0")
+	message("Adding FetchContent_MakeAvailable")
+	# from cmakes sources
+	macro(FetchContent_MakeAvailable)
+
+		foreach(contentName IN ITEMS ${ARGV})
+			string(TOLOWER ${contentName} contentNameLower)
+			FetchContent_GetProperties(${contentName})
+			if(NOT ${contentNameLower}_POPULATED)
+				FetchContent_Populate(${contentName})
+
+				# Only try to call add_subdirectory() if the populated content
+				# can be treated that way. Protecting the call with the check
+				# allows this function to be used for projects that just want
+				# to ensure the content exists, such as to provide content at
+				# a known location.
+				if(EXISTS ${${contentNameLower}_SOURCE_DIR}/CMakeLists.txt)
+					add_subdirectory(${${contentNameLower}_SOURCE_DIR}
+						${${contentNameLower}_BINARY_DIR})
+				endif()
+			endif()
+		endforeach()
+
+	endmacro()
+endif()
+
 include(GNUInstallDirs)
 
 # Include Qt basic functions