Add support for Musl libc (#574)

* Add support for Musl libc

Since Musl is sufficiently different from Glibc (see https://wiki.musl-libc.org/functional-differences-from-glibc.html), it requires a different import, which now should be applied to files that have `import Glibc` in them.

* Platform.swift: fix macOS build issue

Glibc and Darwin can share the same `ioctl` code.

Author: MaxDesiatov

Sources/ArgumentParser/Utilities/Platform.swift

@@ -11,6 +11,8 @@
 
 #if canImport(Glibc)
 import Glibc
+#elseif canImport(Musl)
+import Musl
 #elseif canImport(Darwin)
 import Darwin
 #elseif canImport(CRT)
@@ -78,6 +80,8 @@ extension Platform {
   static func exit(_ code: Int32) -> Never {
 #if canImport(Glibc)
     Glibc.exit(code)
+#elseif canImport(Musl)
+    Musl.exit(code)
 #elseif canImport(Darwin)
     Darwin.exit(code)
 #elseif canImport(CRT)
@@ -135,6 +139,8 @@ extension Platform {
     // TIOCGWINSZ is a complex macro, so we need the flattened value.
     let tiocgwinsz = Int32(0x40087468)
     let err = ioctl(STDOUT_FILENO, tiocgwinsz, &w)
+#elseif canImport(Musl)
+    let err = ioctl(STDOUT_FILENO, UInt(TIOCGWINSZ), &w)
 #else
     let err = ioctl(STDOUT_FILENO, TIOCGWINSZ, &w)
 #endif