cmd/cgo: more robust detection of clang

Fixes #10453.

Change-Id: I77470279865d4c954df615d6594c69edf68c28ca
Reviewed-on: https://go-review.googlesource.com/9090Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/cgo/gcc.go

@@ -199,6 +199,10 @@ func (p *Package) loadDefines(f *File) {
 			val = strings.TrimSpace(line[tabIndex:])
 		}
 
+		if key == "__clang__" {
+			p.GccIsClang = true
+		}
+
 		if n := f.Name[key]; n != nil {
 			if *debugDefine {
 				fmt.Fprintf(os.Stderr, "#define %s %s\n", key, val)
@@ -762,7 +766,7 @@ func (p *Package) gccCmd() []string {
 		"-c",          // do not link
 		"-xc",         // input language is C
 	)
-	if strings.Contains(c[0], "clang") {
+	if p.GccIsClang {
 		c = append(c,
 			"-ferror-limit=0",
 			// Apple clang version 1.7 (tags/Apple/clang-77) (based on LLVM 2.9svn)

src/cmd/cgo/main.go

@@ -33,6 +33,7 @@ type Package struct {
 	PtrSize     int64
 	IntSize     int64
 	GccOptions  []string
+	GccIsClang  bool
 	CgoFlags    map[string][]string // #cgo flags (CFLAGS, LDFLAGS)
 	Written     map[string]bool
 	Name        map[string]*Name // accumulated Name from Files

src/cmd/cgo/out.go

@@ -626,7 +626,7 @@ func (p *Package) writeGccgoOutputFunc(fgcc *os.File, n *Name) {
 // and http://golang.org/issue/5603.
 func (p *Package) packedAttribute() string {
 	s := "__attribute__((__packed__"
-	if !strings.Contains(p.gccBaseCmd()[0], "clang") && (goarch == "amd64" || goarch == "386") {
+	if !p.GccIsClang && (goarch == "amd64" || goarch == "386") {
 		s += ", __gcc_struct__"
 	}
 	return s + "))"