WERROR_COMMON_FLAGS isn't a list, so don't treat it as one.

Change-Id: Iea0b39a61fade5ac87ed31efaf8c1c7a90aa4fb7
Reviewed-on: https://code.wireshark.org/review/29617
Reviewed-by: Guy Harris <guy@alum.mit.edu>

CMakeLists.txt

@@ -750,10 +750,11 @@ if(NOT DISABLE_WERROR AND NOT ENABLE_EXTRA_COMPILER_WARNINGS)
 		# all errors.
 		#
 		if (APPLE AND NOT CMAKE_C_COMPILER_ID MATCHES "Clang")
-			list(APPEND WERROR_COMMON_FLAGS
+			set(WERROR_COMMON_FLAGS
+				"${WERROR_COMMON_FLAGS}
 				-Wno-error=comma
 				-Wno-error=shorten-64-to-32
-				-Wno-error=unused-function
+				-Wno-error=unused-function"
 			)
 		endif()
 	endif()