Updates for CMake 3, CMAKE_MODULE_PATH as subproject

Update minimum required version to 3.16 (supported by most current Linux
distributions). CMake 2 is no longer supported.

CMAKE_MODULE_PATH set with CMAKE_CURRENT_SOURCE_DIR instead of
CMAKE_SOURCE_DIR -- this enables libcbor use in an add_subdirectory
context, e.g. with CMake FetchContent.

subdirs is deprecated in favor of add_subdirectory.

CHANGELOG.md

@@ -1,6 +1,8 @@
 Next
 ---------------------
 
+- Minimum CMake version is 3.16 [[#201]](https://github.com/PJK/libcbor/pull/201) (by [thewtex@](https://github.com/thewtex))
+
 0.9.0 (2021-11-14)
 ---------------------
 - Improved pkg-config paths handling [[#164]](https://github.com/PJK/libcbor/pull/164) (by [jtojnar@](https://github.com/jtojnar))

CMakeLists.txt

@@ -1,6 +1,6 @@
-cmake_minimum_required(VERSION 2.8)
+cmake_minimum_required(VERSION 3.16)
 project(libcbor)
-set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_SOURCE_DIR}/CMakeModules/")
+set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_CURRENT_SOURCE_DIR}/CMakeModules/")
 include(CTest)
 
 SET(CBOR_VERSION_MAJOR "0")
@@ -139,20 +139,20 @@ else()
     message(STATUS "LTO is not enabled")
 endif(use_lto)
 
-subdirs(src)
+add_subdirectory(src)
 if(use_lto)
     set_property(DIRECTORY src PROPERTY INTERPROCEDURAL_OPTIMIZATION TRUE)
 endif(use_lto)
 
 if (WITH_TESTS)
-    subdirs(test)
+    add_subdirectory(test)
     if(use_lto)
         set_property(DIRECTORY test PROPERTY INTERPROCEDURAL_OPTIMIZATION TRUE)
     endif(use_lto)
 endif (WITH_TESTS)
 
 if (WITH_EXAMPLES)
-    subdirs(examples)
+    add_subdirectory(examples)
     if(use_lto)
         set_property(DIRECTORY examples PROPERTY INTERPROCEDURAL_OPTIMIZATION TRUE)
     endif(use_lto)