1. -Wno-unused-private-field -Wno-unknown-warning-option don't exist on gcc 4.8
2. Respect CXXFLAGS
Index: src/CMakeLists.txt
--- src/CMakeLists.txt.orig
+++ src/CMakeLists.txt
@@ -1,6 +1,6 @@
 
 set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wall")
-set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Woverloaded-virtual -Wall -Wno-sign-compare -Wno-deprecated-declarations -Wno-unused-local-typedefs -Wno-unused-private-field -Wno-unknown-warning-option")
+set(CMAKE_CXX_FLAGS "-Woverloaded-virtual -Wall -Wno-sign-compare -Wno-deprecated-declarations -Wno-unused-local-typedefs ${CMAKE_CXX_FLAGS}")
 
 option(BUILD_WERROR "Build with -Werror" ON)