Default to macOS 10.10 minimum deployment target again but allow it to be overridden

This reverts commit 5e968d4 since it caused breakage, but keeps the iOS 13 minimum deployment target.

Author: abertelrud

Package.swift

@@ -14,10 +14,17 @@
 import PackageDescription
 import class Foundation.ProcessInfo
 
+let macOSPlatform: SupportedPlatform
+if let deploymentTarget = ProcessInfo.processInfo.environment["SWIFTTSC_MACOS_DEPLOYMENT_TARGET"] {
+    macOSPlatform = .macOS(deploymentTarget)
+} else {
+    macOSPlatform = .macOS(.v10_10)
+}
+
 let package = Package(
     name: "swift-tools-support-core",
     platforms: [
-        .macOS(.v10_15),
+        macOSPlatform,
         .iOS(.v13)
     ],
     products: [