Add check before using a flag to ensure clang can compile

CMakeLists.txt

@@ -117,7 +117,11 @@ if (MORSE_DISTRIB_DIR OR EXISTS "${CMAKE_SOURCE_DIR}/CMakeModules/morse/")
   else()
     # NOT      INTEL
     if(NOT ScalFMM_USE_MPI)
-      set(ScalFMM_CXX_FLAGS  "${ScalFMM_CXX_FLAGS} -Wzero-as-null-pointer-constant")
+      include(CheckCCompilerFlag)
+      check_c_compiler_flag(-Wzero-as-null-pointer-constant HAS_WZERO_NULL_PTR_FLAG)
+      if(HAS_WZERO_NULL_PTR_FLAG)
+        set(ScalFMM_CXX_FLAGS  "${ScalFMM_CXX_FLAGS} -Wzero-as-null-pointer-constant")
+      endif()
     else()
       include(CheckCCompilerFlag)
       check_c_compiler_flag(-Wno-literal-suffix HAS_NO_LITERAL_SUFFIX_FLAG)