cmd/go: build SWIG shared libraries in work directory

Remove test of whether SWIG shared library is older than
sources--should be covered by test of package file anyhow.

Fixes #5739.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/13352046

src/cmd/go/build.go

@@ -1005,8 +1005,9 @@ func (b *builder) install(a *action) (err error) {
 				return err
 			}
 			soname := a.p.swigSoname(f)
+			source := filepath.Join(a.objdir, soname)
 			target := filepath.Join(dir, soname)
-			if err = b.copyFile(a, target, soname, perm); err != nil {
+			if err = b.copyFile(a, target, source, perm); err != nil {
 				return err
 			}
 		}
@@ -2255,7 +2256,8 @@ func (b *builder) swigOne(p *Package, file, obj string, cxx bool, intgosize stri
 		cxxlib = []string{"-lstdc++"}
 	}
 	ldflags := stringList(osldflags[goos], cxxlib)
-	b.run(p.Dir, p.ImportPath, nil, b.gccCmd(p.Dir), "-o", soname, gccObj, ldflags)
+	target := filepath.Join(obj, soname)
+	b.run(p.Dir, p.ImportPath, nil, b.gccCmd(p.Dir), "-o", target, gccObj, ldflags)
 
 	return obj + goFile, cObj, nil
 }

src/cmd/go/pkg.go

@@ -680,28 +680,13 @@ func isStale(p *Package, topRoot map[string]bool) bool {
 		return false
 	}
 
-	srcs := stringList(p.GoFiles, p.CFiles, p.CXXFiles, p.HFiles, p.SFiles, p.CgoFiles, p.SysoFiles)
+	srcs := stringList(p.GoFiles, p.CFiles, p.CXXFiles, p.HFiles, p.SFiles, p.CgoFiles, p.SysoFiles, p.SwigFiles, p.SwigCXXFiles)
 	for _, src := range srcs {
 		if olderThan(filepath.Join(p.Dir, src)) {
 			return true
 		}
 	}
 
-	for _, src := range stringList(p.SwigFiles, p.SwigCXXFiles) {
-		if olderThan(filepath.Join(p.Dir, src)) {
-			return true
-		}
-		soname := p.swigSoname(src)
-		fi, err := os.Stat(soname)
-		if err != nil {
-			return true
-		}
-		fiSrc, err := os.Stat(src)
-		if err != nil || fiSrc.ModTime().After(fi.ModTime()) {
-			return true
-		}
-	}
-
 	return false
 }