Support compiling against Musl (#145)

Author: simonjbeaumont

IntegrationTests/allocation-counter-tests-framework/template/scaffolding.swift

@@ -28,10 +28,14 @@
 
 import AtomicCounter
 import Foundation
-#if os(macOS) || os(iOS) || os(watchOS) || os(tvOS)
+#if canImport(Darwin)
 import Darwin
-#else
+#elseif canImport(Glibc)
 import Glibc
+#elseif canImport(Musl)
+import Musl
+#else
+#error("Unsupported runtime")
 #endif
 
 func waitForThreadsToQuiesce(shouldReachZero: Bool) {

Sources/Instrumentation/Locks.swift

@@ -26,10 +26,14 @@
 //
 //===----------------------------------------------------------------------===//
 
-#if os(macOS) || os(iOS) || os(tvOS) || os(watchOS)
+#if canImport(Darwin)
 import Darwin
-#else
+#elseif canImport(Glibc)
 import Glibc
+#elseif canImport(Musl)
+import Musl
+#else
+#error("Unsupported runtime")
 #endif
 
 /// A threading lock based on `libpthread` instead of `libdispatch`.

Sources/Tracing/TracingTime.swift

@@ -12,10 +12,14 @@
 //
 //===----------------------------------------------------------------------===//
 
-#if os(Linux)
+#if canImport(Darwin)
+import Darwin
+#elseif canImport(Glibc)
 import Glibc
+#elseif canImport(Musl)
+import Musl
 #else
-import Darwin
+#error("Unsupported runtime")
 #endif
 
 @_exported import Instrumentation