No fatal on linux Swift < 5

Sources/Path+ls.swift

@@ -130,7 +130,11 @@ public extension Path.Finder {
         while let path = next() {
             switch try closure(path) {
             case .skip:
+              #if !os(Linux) || swift(>=5.0)
                 enumerator.skipDescendants()
+              #else
+                fputs("warning: skip is not implemented for Swift < 5.0\n", stderr)
+              #endif
             case .abort:
                 return
             case .continue:

Tests/PathTests/PathTests+ls().swift

@@ -169,7 +169,7 @@ extension PathTests {
             try tmpdir.b.mkdir().join("c").touch()
             try tmpdir.b.d.mkdir().join("e").touch()
             try tmpdir.b.d.f.mkdir().join("g").touch()
-
+          #if !os(Linux) || swift(>=5)
             do {
                 var rv = Set<Path>()
 
@@ -184,6 +184,7 @@ extension PathTests {
 
                 XCTAssertEqual(rv, Set([tmpdir.a, tmpdir.b, tmpdir.b.c].map(Path.init)))
             }
+          #endif
             do {
                 var x = 0