Don't use CXXFLAGS when compiling eh_trampoline.cc

We don't want the user to override these flags because that file
needs to be compiled in a very specific way.

Re-applies gnustep#178 (365e536)
Related to gnustep#183
Fixes gnustep#177

CMakeLists.txt

@@ -230,7 +230,7 @@ elseif (MINGW)
 	message(STATUS "Using MinGW-compatible exception model")
 	list(APPEND libobjc_CXX_SRCS objcxx_eh.cc objcxx_eh_mingw.cc)
 else ()
-	separate_arguments(EH_PERSONALITY_FLAGS NATIVE_COMMAND ${CMAKE_CXX_FLAGS})
+	set(EH_PERSONALITY_FLAGS "")
 	if (CMAKE_CXX_COMPILER_TARGET)
 		list(APPEND EH_PERSONALITY_FLAGS "${CMAKE_CXX_COMPILE_OPTIONS_TARGET}${CMAKE_CXX_COMPILER_TARGET}")
 	endif ()