build: fix swift detection in major swift versions

the swift version string on major versions only has two components,
major and minor, the third one is missing.

waftools/detections/compiler_swift.py

@@ -15,7 +15,7 @@ def __add_swift_flags(ctx):
     ctx.env.SWIFT_FLAGS = ('-frontend -c -sdk %s -enable-objc-interop -emit-objc-header'
                            ' -emit-module -parse-as-library') % (ctx.env.MACOS_SDK)
     swift_version = __run(ctx.env.SWIFT +  ' -version').split(' ')[3].split('.')
-    major, minor, sub = [int(n) for n in swift_version]
+    major, minor = [int(n) for n in swift_version]
 
     # the -swift-version parameter is only supported on swift 3.1 and newer
     if major >= 3 and minor >= 1 or major >= 4: