Supress warnings from external package dCache

Also remove add_definitions(-pthread) from CMakeLists.txt.
This is not the right function to use to add the flag, and
it is already taken care of by cmake/modules/CheckCompiler.cmake
in any case.

io/dcache/CMakeLists.txt

@@ -2,10 +2,7 @@
 # CMakeLists.txt file for building ROOT io/dcache package
 ############################################################################
 
-include_directories(${DCAP_INCLUDE_DIRS})
-add_definitions(-pthread)
-
-ROOT_ADD_CXX_FLAG(CMAKE_CXX_FLAGS -Wno-ignored-qualifiers)
+include_directories(SYSTEM ${DCAP_INCLUDE_DIRS})
 
 ROOT_GENERATE_DICTIONARY(G__DCache *.h MODULE DCache LINKDEF LinkDef.h)