cmake: Remove -Wcomma for C++ when using Qt >= 5.10

The Qt team introduced a lot of "Possible misuse of comma operator here"
warnings in qstring.h and qstringview.h in version 5.10.  Now we need to
fix the Qt source again.

Change-Id: I948ba5a224d4dca1ebb6a758100c064d501c1d4a
Reviewed-on: https://code.wireshark.org/review/24779
Petri-Dish: Stig Bjørlykke <stig@bjorlykke.org>
Tested-by: Petri Dish Buildbot
Reviewed-by: Stig Bjørlykke <stig@bjorlykke.org>

CMakeLists.txt

@@ -464,7 +464,6 @@ else()
 		-Wno-overlength-strings
 		-Wno-long-long
 		-Wheader-guard
-		-Wcomma
 	)
 
 	#
@@ -517,6 +516,11 @@ else()
 		# -Wimplicit-function-declaration into an error by default.
 		#
 		-Werror=implicit
+		#
+		# The Qt headers in version 5.10 introduced a lot of
+		# "Possible misuse of comma operator here" warnings.
+		#
+		-Wcomma
 	)
 
 	set(CXX_WARN_FLAGS
@@ -530,6 +534,13 @@ else()
 			-Wshorten-64-to-32
 		)
 	endif()
+	if (Qt5Core_VERSION VERSION_LESS 5.10)
+		# The Qt headers in version 5.10 introduced a lot of
+		# "Possible misuse of comma operator here" warnings.
+		set(CXX_WARN_FLAGS ${CXX_WARN_FLAGS}
+			-Wcomma
+		)
+	endif()
 
 	#
 	# These are not enabled by default, because the warnings they