Use established constants for version checks

.luacheckrc

@@ -55,6 +55,9 @@ read_globals = {
 
 	-- FrameXML constants
 	'DEFAULT_CHAT_FRAME',
+    'WOW_PROJECT_ID',
+    'WOW_PROJECT_MAINLINE',
+    'WOW_PROJECT_CLASSIC',
 
 	-- GlobalStrings
 	'HEADER_COLON',

modules/build.lua

@@ -1,27 +1,29 @@
 local _, addon = ...
 
-local _, buildVersion, _, interfaceVersion = GetBuildInfo()
 --[[ namespace:IsRetail()
 Checks if the current client is running the "retail" version.
 --]]
 function addon:IsRetail()
-	return interfaceVersion > 100000
+	return WOW_PROJECT_ID == WOW_PROJECT_MAINLINE
 end
 
 --[[ namespace:IsClassicEra()
 Checks if the current client is running the "classic era" version (e.g. vanilla).
 --]]
 function addon:IsClassicEra()
-	return interfaceVersion < 20000
+	return WOW_PROJECT_ID == WOW_PROJECT_CLASSIC
 end
 
 --[[ namespace:IsClassic()
 Checks if the current client is running the "classic" version.
 --]]
 function addon:IsClassic()
+	-- instead of using the floating constant for classic we'll just NOR the other two,
+	-- as they are static
 	return not addon:IsRetail() and not addon:IsClassicEra()
 end
 
+local _, buildVersion, _, interfaceVersion = GetBuildInfo()
 --[[ namespace:HasBuild(_buildNumber_[, _interfaceVersion_])
 Checks if the current client is running a build equal to or newer than the specified.  
 Optionally also check against the interface version.