cmake: fix build with 3.16

thirdparty/cpm-imgui.cmake

@@ -21,6 +21,7 @@ if(imgui_ADDED)
 
 	add_custom_command(
 		OUTPUT "${imgui_BINARY_DIR}/include/imgui.h" "${imgui_BINARY_DIR}/include/imconfig.h"
+		COMMAND ${CMAKE_COMMAND} -E make_directory "${imgui_BINARY_DIR}/include"
 		COMMAND ${CMAKE_COMMAND} -E copy "${imgui_SOURCE_DIR}/imgui.h" "${imgui_SOURCE_DIR}/imconfig.h" "${imgui_BINARY_DIR}/include"
 		DEPENDS "${imgui_SOURCE_DIR}/imgui.h" "${imgui_SOURCE_DIR}/imconfig.h"
 		VERBATIM

thirdparty/tcbrindle_span/CMakeLists.txt

@@ -2,7 +2,10 @@
 # Portable implementation of C++20 std::span
 # Boost License 1.0
 
-add_library(tcbrindle_span INTERFACE include/tcb/span.hpp)
+cmake_minimum_required(VERSION 3.14 FATAL_ERROR)
+
+add_library(tcbrindle_span INTERFACE)
+target_sources(tcbrindle_span INTERFACE include/tcb/span.hpp)
 target_include_directories(tcbrindle_span INTERFACE include)
 
 add_library(tcbrindle::span ALIAS tcbrindle_span)