Avoid using separate '-D' + '...' flags in CFLAGS in CMake

[kubamracek]

Please look at the code change, and think "how could this possibly be a significant change?". If you come up empty, like me when I originally hit this, then click here to reveal the solution. CMake 🤷

[al45tair]

Aaaargh!

[mikeash]

I'm pretty sure I've seen this before. Also, WTF CMake?!

[nate-chandler]

Error, your directive was too legible. -CMake, probably

[mikeash]

Proposed:

@@ -55,9 +55,9 @@ public enum CommandLine {
-  public static var arguments: [String]
+  public static var arguments: Set<String>

[kubamracek]

@swift-ci please test

[compnerd]

This isn't really that weird. It actually is a positive. The point is that people often repeat the options on the command line, CMake will deduplicate it to get a saner command line.

[kubamracek]

@swift-ci please test Linux platform

[kubamracek]

@swift-ci please test Linux platform