Don't use CXXFLAGS when compiling eh_trampoline.cc

The way that we were doing this didn't handle multiple flags and we
actually don't want the user to override these flags because that file
needs to be compiled in a very specific way.

Fixes #177

CMakeLists.txt

@@ -317,7 +317,7 @@ if (ENABLE_OBJCXX)
 			endif()
 		endif ()
 		add_custom_command(OUTPUT eh_trampoline.s
-			COMMAND ${CMAKE_CXX_COMPILER} ${CMAKE_CXX_FLAGS} -fPIC -S "${CMAKE_SOURCE_DIR}/eh_trampoline.cc" -o - -fexceptions -fno-inline | sed "s/__gxx_personality_v0/test_eh_personality/g" > "${CMAKE_BINARY_DIR}/eh_trampoline.s"
+			COMMAND ${CMAKE_CXX_COMPILER} -fPIC -S "${CMAKE_SOURCE_DIR}/eh_trampoline.cc" -o - -fexceptions -fno-inline | sed "s/__gxx_personality_v0/test_eh_personality/g" > "${CMAKE_BINARY_DIR}/eh_trampoline.s"
 			MAIN_DEPENDENCY eh_trampoline.cc)
 		list(APPEND libobjc_ASM_SRCS eh_trampoline.s)
 		list(APPEND libobjc_CXX_SRCS objcxx_eh.cc)