builder: Make paths absolute for stdliblist (bazel-contrib#1357)

Make the gopackagesdriver work when using a C/C++ toolchain with
sysroots. We need to turn the relative in flag like --sysroot into
an absolute.

Address the todo by using the code already available in the stdlib.go
as part of bazel-contrib#1536.

Author: zecke

go/tools/builders/stdlib.go

@@ -81,7 +81,6 @@ You may need to use the flags --cpu=x64_windows --compiler=mingw-gcc.`)
 	os.Setenv("GO111MODULE", "off")
 
 	// 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", quotePathIfNeeded(abs(os.Getenv("CC"))))
 
 	// Ensure paths are absolute.
@@ -159,7 +158,7 @@ You may need to use the flags --cpu=x64_windows --compiler=mingw-gcc.`)
 	installArgs = append(installArgs, "-ldflags="+allSlug+strings.Join(ldflags, " "))
 	installArgs = append(installArgs, "-asmflags="+allSlug+strings.Join(asmflags, " "))
 
-	// Modifying CGO flags to use only absolute path
+	// Modify CGO flags to use only absolute path
 	// because go is having its own sandbox, all CGO flags must use absolute path
 	if err := absEnv(cgoEnvVars, cgoAbsEnvFlags); err != nil {
 		return fmt.Errorf("error modifying cgo environment to absolute path: %v", err)

go/tools/builders/stdliblist.go

@@ -251,13 +251,18 @@ func stdliblist(args []string) error {
 
 	cgoEnabled := os.Getenv("CGO_ENABLED") == "1"
 	// Make sure we have an absolute path to the C compiler.
-	// TODO(#1357): also take absolute paths of includes and other paths in flags.
 	ccEnv, ok := os.LookupEnv("CC")
 	if cgoEnabled && !ok {
 		return fmt.Errorf("CC must be set")
 	}
 	os.Setenv("CC", quotePathIfNeeded(abs(ccEnv)))
 
+	// Modify CGO flags to use only absolute path
+	// because go is having its own sandbox, all CGO flags must use absolute path
+	if err := absEnv(cgoEnvVars, cgoAbsEnvFlags); err != nil {
+		return fmt.Errorf("error modifying cgo environment to absolute path: %v", err)
+	}
+
 	// We want to keep the cache around so that the processed files can be used by other tools.
 	absCachePath := abs(*cachePath)
 	os.Setenv("GOCACHE", absCachePath)