build(libnetlink): allow setting CMAKE_BUILD_TYPE

Previously, libnetlink was harded to compile in DEBUG mode.
Now it should inherit EDGESec's compile mode.

lib/libnetlink/CMakeLists.txt

@@ -12,7 +12,14 @@ endif()
 # libmnl0 linking doesn't work properly, unless we setup RPATHs
 SET(CMAKE_INSTALL_RPATH "$ORIGIN" "$<TARGET_LINKER_FILE_DIR:MNL::mnl>")
 
-set(CMAKE_BUILD_TYPE Debug)
+# Set a default build type if none was specified
+if(NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)
+  message(STATUS "Setting build type to 'Debug' as none was specified.")
+  set(CMAKE_BUILD_TYPE Debug CACHE STRING "Choose the type of build." FORCE)
+  # Set the possible values of build type for cmake-gui
+  set_property(CACHE CMAKE_BUILD_TYPE PROPERTY STRINGS "Debug" "Release"
+    "MinSizeRel" "RelWithDebInfo")
+endif()
 
 set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -D_GNU_SOURCE")
 set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -DHAVE_LIBMNL")