all: add Nix flake file

This adds a flake.nix file that makes it possible to quickly create a
development environment.

You can download Nix here, for use on your Linux or macOS system:
https://nixos.org/download.html

After you have installed Nix, you can enter the development environment
as follows:

    nix develop

This drops you into a bash shell, where you can install TinyGo simply
using the following command:

    go install

That's all! Assuming you've set up your $PATH correctly, you can now use
the tinygo command as usual:

    tinygo version

You can also do many other things from this environment. Building and
flashing should work as you're used to: it's not a VM or container so
there are no access restrictions.

compileopts/config.go

@@ -275,7 +275,8 @@ func (c *Config) CFlags(libclang bool) []string {
 	case "darwin-libSystem":
 		root := goenv.Get("TINYGOROOT")
 		cflags = append(cflags,
-			"--sysroot="+filepath.Join(root, "lib/macos-minimal-sdk/src"),
+			"-nostdlibinc",
+			"-isystem", filepath.Join(root, "lib/macos-minimal-sdk/src/usr/include"),
 		)
 	case "picolibc":
 		root := goenv.Get("TINYGOROOT")
@@ -304,7 +305,8 @@ func (c *Config) CFlags(libclang bool) []string {
 		root := goenv.Get("TINYGOROOT")
 		path, _ := c.LibcPath("mingw-w64")
 		cflags = append(cflags,
-			"--sysroot="+path,
+			"-nostdlibinc",
+			"-isystem", filepath.Join(path, "include"),
 			"-isystem", filepath.Join(root, "lib", "mingw-w64", "mingw-w64-headers", "crt"),
 			"-isystem", filepath.Join(root, "lib", "mingw-w64", "mingw-w64-headers", "defaults", "include"),
 			"-D_UCRT",