cmd/cgo: add -srcdir option

This is convenient for direct use of `go tool cgo`. We can also use it
from the go tool to reduce the length of the file names that cgo
generates.

Update #17070.

Change-Id: I8466a0a2cc68a732d17d07319e303497715bac8c
Reviewed-on: https://go-review.googlesource.com/32354
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/cgo/doc.go

@@ -326,6 +326,9 @@ The following options are available when running cgo directly:
 		Write out input file in Go syntax replacing C package
 		names with real values. Used to generate files in the
 		syscall package when bootstrapping a new target.
+	-srcdir directory
+		Find the Go input files, listed on the command line,
+		in directory.
 	-objdir directory
 		Put all generated files in directory.
 	-importpath string

src/cmd/cgo/main.go

@@ -178,6 +178,7 @@ var dynlinker = flag.Bool("dynlinker", false, "record dynamic linker information
 // constant values used in the host's C libraries and system calls.
 var godefs = flag.Bool("godefs", false, "for bootstrap: write Go definitions for C file to standard output")
 
+var srcDir = flag.String("srcdir", "", "source directory")
 var objDir = flag.String("objdir", "", "object directory")
 var importPath = flag.String("importpath", "", "import path of package being built (for comments in generated files)")
 var exportHeader = flag.String("exportheader", "", "where to write export header if any exported functions")
@@ -256,6 +257,9 @@ func main() {
 	// Use the beginning of the md5 of the input to disambiguate.
 	h := md5.New()
 	for _, input := range goFiles {
+		if *srcDir != "" {
+			input = filepath.Join(*srcDir, input)
+		}
 		f, err := os.Open(input)
 		if err != nil {
 			fatalf("%s", err)
@@ -267,6 +271,9 @@ func main() {
 
 	fs := make([]*File, len(goFiles))
 	for i, input := range goFiles {
+		if *srcDir != "" {
+			input = filepath.Join(*srcDir, input)
+		}
 		f := new(File)
 		f.ReadGo(input)
 		f.DiscardCgoDirectives()