Quote CC if it contains whitespace (#3491)

The `cgo` tool splits `CC` on unquoted whitespace, which can cause
failures on Windows when `CC` points to an absolute path such as
`C:\Program Files\some\compiler.exe`.

go/tools/builders/env.go

@@ -436,6 +436,23 @@ func passLongArgsInResponseFiles(cmd *exec.Cmd) (cleanup func()) {
 	return cleanup
 }
 
+// quotePathIfNeeded quotes path if it contains whitespace and isn't already quoted.
+// Use this for paths that will be passed through
+// https://github.com/golang/go/blob/06264b740e3bfe619f5e90359d8f0d521bd47806/src/cmd/internal/quoted/quoted.go#L25
+func quotePathIfNeeded(path string) string {
+	if strings.HasPrefix(path, "\"") || strings.HasPrefix(path, "'") {
+		// Assume already quoted
+		return path
+	}
+	// https://github.com/golang/go/blob/06264b740e3bfe619f5e90359d8f0d521bd47806/src/cmd/internal/quoted/quoted.go#L16
+	if strings.IndexAny(path, " \t\n\r") < 0 {
+		// Does not require quoting
+		return path
+	}
+	// Escaping quotes is not supported, so we can assume path doesn't contain any quotes.
+	return "'" + path + "'"
+}
+
 func useResponseFile(path string, argLen int) bool {
 	// Unless the program uses objabi.Flagparse, which understands
 	// response files, don't use response files.

go/tools/builders/stdlib.go

@@ -79,7 +79,7 @@ You may need to use the flags --cpu=x64_windows --compiler=mingw-gcc.`)
 
 	// Make sure we have an absolute path to the C compiler.
 	// TODO(#1357): also take absolute paths of includes and other paths in flags.
-	os.Setenv("CC", abs(os.Getenv("CC")))
+	os.Setenv("CC", quotePathIfNeeded(abs(os.Getenv("CC"))))
 
 	// Ensure paths are absolute.
 	absPaths := []string{}

go/tools/builders/stdliblist.go

@@ -202,7 +202,7 @@ func stdliblist(args []string) error {
 	os.Setenv("GOROOT", newGoRoot)
 	// Make sure we have an absolute path to the C compiler.
 	// TODO(#1357): also take absolute paths of includes and other paths in flags.
-	os.Setenv("CC", abs(os.Getenv("CC")))
+	os.Setenv("CC", quotePathIfNeeded(abs(os.Getenv("CC"))))
 
 	cachePath := abs(*out + ".gocache")
 	defer os.RemoveAll(cachePath)