cmd/dist: only run swig tests when a go directory is present in swiglib

When there is no go directory inside the swiglib directory then swig
was installed without Go support. Tests in misc/swig will fail when
swig is installed without Go support.

Add additional checks for the presence of a go directory in the directory
reported by 'swig -go -swiglib' to determine if misc/swig tests should
be run.

This avoids all.bash failing when swig but not swig-go is installed
using macports.

Tested on darwin with swig and with and without swig-go installed
using macports.

Fixes #23469

Change-Id: I173201221554982ea0d9f2bea70a3cb85b297cec
Reviewed-on: https://go-review.googlesource.com/88776Reviewed-by: David Chase <drchase@google.com>

src/cmd/dist/test.go

@@ -1210,6 +1210,23 @@ func (t *tester) hasSwig() bool {
 	if err != nil {
 		return false
 	}
+
+	// Check that swig was installed with Go support by checking
+	// that a go directory exists inside the swiglib directory.
+	// See https://golang.org/issue/23469.
+	output, err := exec.Command(swig, "-go", "-swiglib").Output()
+	if err != nil {
+		return false
+	}
+	swigDir := strings.TrimSpace(string(output))
+
+	_, err = os.Stat(filepath.Join(swigDir, "go"))
+	if err != nil {
+		return false
+	}
+
+	// Check that swig has a new enough version.
+	// See https://golang.org/issue/22858.
 	out, err := exec.Command(swig, "-version").CombinedOutput()
 	if err != nil {
 		return false