Add CMake logic to support both IDF v3.3 and v4.x.

CMakeLists.txt

@@ -5,5 +5,15 @@ cmake_minimum_required(VERSION 3.5)
 include($ENV{IDF_PATH}/tools/cmake/project.cmake)
 project(esp32-ds18b20-example)
 
+if (NOT IDF_VERSION_MAJOR)
+  set(IDF_VERSION_MAJOR 3)
+endif()
+
+if (IDF_VERSION_MAJOR GREATER 3)
+  set(PROJECT_ELF ${project_elf})
+else()
+  set(PROJECT_ELF ${IDF_PROJECT_EXECUTABLE})
+endif()
+
 # Ignore false clang warnings about `struct foo = { 0 }`
-target_compile_options(${project_elf} PRIVATE -Wno-missing-braces -Wmissing-field-initializers)
+target_compile_options(${PROJECT_ELF} PRIVATE -Wno-missing-braces -Wmissing-field-initializers)