SWDEV-272485 append to CMAKE_CXX_FLAGS instead of override

Change-Id: I0c4364fa800a249d77ec7abd4e73556a8df1ef99

cmake_modules/env.cmake

@@ -36,7 +36,7 @@ add_definitions ( -DHSA_LARGE_MODEL= )
 add_definitions ( -DHSA_DEPRECATED= )
 
 ## Linux Compiler options
-set ( CMAKE_CXX_FLAGS "-std=c++11")
+set ( CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
 set ( CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall" )
 set ( CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Werror" )
 set ( CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Werror=return-type" )