Initial CMake files for pico-sdk (#174)

* Add CMakeLists.txt that defines RF24Network as an INTERFACE library and works at least with the pico-sdk

* Change the CMake-mechanism to match what we have in the RF24 library

* Rename files so it might work with older versions of cmake as well

CMakeLists.txt

@@ -0,0 +1,9 @@
+# Check if we are building a pico-sdk based project
+# (or more exactly: if we just got included in a pico-sdk based project)
+if (PICO_SDK_PATH)
+    # If so, load the relevant CMakeLists-file but don't do anything else
+    include(${CMAKE_CURRENT_LIST_DIR}/usePicoSDK.cmake)
+    return()
+endif()
+
+## TODO: Non-PicoSDK builds

usePicoSDK.cmake

@@ -0,0 +1,18 @@
+## Include this file if you want to use the RF24Network library
+## in YOUR (pico-sdk based) project.
+
+cmake_minimum_required(VERSION 3.12)
+
+set(CMAKE_C_STANDARD 11)
+set(CMAKE_CXX_STANDARD 17)
+
+# Define the RF24Network library
+add_library(RF24Network INTERFACE)
+
+target_sources(RF24Network INTERFACE
+    ${CMAKE_CURRENT_LIST_DIR}/RF24Network.cpp
+)
+
+target_include_directories(RF24Network INTERFACE
+    ${CMAKE_CURRENT_LIST_DIR}/
+)