Respect CXXFLAGS

Index: tests/CMakeLists.txt
--- tests/CMakeLists.txt.orig
+++ tests/CMakeLists.txt
@@ -1,6 +1,6 @@
 cmake_minimum_required(VERSION 2.8.11)
 
-set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fpermissive -Wno-c++11-narrowing")
+set(CMAKE_CXX_FLAGS "-fpermissive -Wno-c++11-narrowing -${CMAKE_CXX_FLAGS}")
 
 if(USE_SYSTEM_GMOCK)
   include_directories(${GMOCK_INCLUDE_DIRS} ${GTEST_INCLUDE_DIRS})
