dbus: fix clang compile errors

This commit downgrades the following clang compiler errors into warnings:
 * unused-private-field
 * unused-const-variable
 * unused-lambda-capture

CMakeLists.txt

@@ -24,6 +24,12 @@ set(C_AND_CXX_WARNINGS "-pedantic -Wall -Wextra")
 set(EXTRA_C_WARNINGS "-Wcast-align -Wcast-qual -Wformat -Wredundant-decls -Wswitch-default")
 set(EXTRA_CXX_WARNINGS "-Wnon-virtual-dtor -Wold-style-cast")
 
+if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "Clang")
+  set(EXTRA_CXX_WARNINGS "${EXTRA_CXX_WARNINGS} -Wno-error=unused-private-field")
+  set(EXTRA_CXX_WARNINGS "${EXTRA_CXX_WARNINGS} -Wno-error=unused-const-variable")
+  set(EXTRA_CXX_WARNINGS "${EXTRA_CXX_WARNINGS} -Wno-error=unused-lambda-capture")
+endif()
+
 set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${C_AND_CXX_WARNINGS}")
 set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${EXTRA_C_WARNINGS}")