Fix missing comma in SConstruct test

The comma was omitted to discourage putting anything after the last
element.  This was fine when there were multiple elements, but with only
one value, the missing comma causes this not to be a tuple as intended.
Add a comma to fix the test.  Add some additional comments on supported
C++ versions by gcc major version.

SConstruct

@@ -760,31 +760,45 @@ help:assume C++ compiler works
 	implicit_tests.append(_implicit_test.RecordedTest('check_cxx14', "assume C++ compiler supports C++14"))
 	__cxx_conformance_CXXFLAGS = [None]
 	def _check_cxx_conformance_level(self,context,_levels=(
-			# List standards in descending order of preference
-			# C++14 is required, so list it last.  Omit the comma as a
-			# reminder not to append elements to the list.
-			_cxx_conformance_cxx14
+			# List standards in descending order of preference.
+			#
+			# Currently, no C++17 features are both useful and easy to
+			# emulate in supported older compilers, so C++17 is not
+			# requested.
+			#
+			# C++14 is required, so list it last.
+			_cxx_conformance_cxx14,
 		), _CXXFLAGS=__cxx_conformance_CXXFLAGS,
 		_successflags={'CXXFLAGS' : __cxx_conformance_CXXFLAGS}
 		):
 		# Testing the compiler option parser only needs Compile, even when LTO
 		# is enabled.
 		Compile = self._Compile
-		# GCC started with -std=gnu++0x for C++0x (later C++11).
-		#
 		# Accepted options by version:
 		#
-		#	gcc-4.9 -std=gnu++0x
-		#	gcc-4.9 -std=gnu++11
 		#	gcc-4.9 -std=gnu++1y
 		#	gcc-4.9 -std=gnu++14
 		#
-		#	gcc-5 -std=gnu++0x
-		#	gcc-5 -std=gnu++11
 		#	gcc-5 -std=gnu++1y
 		#	gcc-5 -std=gnu++14
 		#	gcc-5 -std=gnu++1z
 		#	gcc-5 -std=gnu++17
+		#
+		#	gcc-6 -std=gnu++1y
+		#	gcc-6 -std=gnu++14
+		#	gcc-6 -std=gnu++1z
+		#	gcc-6 -std=gnu++17
+		#
+		#	gcc-7 -std=gnu++1y
+		#	gcc-7 -std=gnu++14
+		#	gcc-7 -std=gnu++1z
+		#	gcc-7 -std=gnu++17
+		#
+		#	gcc-8 -std=gnu++1y
+		#	gcc-8 -std=gnu++14
+		#	gcc-8 -std=gnu++1z
+		#	gcc-8 -std=gnu++17
+		#	gcc-8 -std=gnu++2a
 		for level in _levels:
 			opt = '-std=gnu++%u' % level
 			_CXXFLAGS[0] = opt