Add -Wduplicated-branches to extra warnings

Change-Id: I11d60b4405f4fde039affcdeebb13a3ceb2aded6
Reviewed-on: https://code.wireshark.org/review/20428
Petri-Dish: Peter Wu <peter@lekensteyn.nl>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>
Reviewed-by: Michael Mann <mmann78@netscape.net>

CMakeLists.txt

@@ -494,6 +494,10 @@ else()
 		# No longer supported by El Capitan clang on C++
 		#
 		-fno-delete-null-pointer-checks
+
+		# Works only with GCC 7
+		-Wduplicated-branches
+
 	)
 
 	set(C_EXTRA_WARN_FLAGS

configure.ac

@@ -748,6 +748,10 @@ AC_ARG_ENABLE(extra-compiler-warnings,
 		# never enable this one with -Werror.
 		#
 		AC_WIRESHARK_COMPILER_FLAGS_CHECK(-Wbad-function-cast, C)
+
+		# Works only with GCC 7
+		AC_WIRESHARK_COMPILER_FLAGS_CHECK(-Wduplicated-branches)
+
 	fi
 ])