fix: include {arch} in default -o filename

This was a recommended way to use rsrc for long time, but never made default
nor explained anywhere (other than in github issues). Given that I'm fixing
some bugs anyway in rsrc now, let's use this time to also make this more user
friendly.

README.txt

@@ -4,7 +4,7 @@ INSTALL: go get github.com/akavel/rsrc
 
 USAGE:
 
-rsrc.exe [-manifest FILE.exe.manifest] [-ico FILE.ico[,FILE2.ico...]] -o FILE.syso
+rsrc.exe [-manifest FILE.exe.manifest] [-ico FILE.ico[,FILE2.ico...]] [OPTIONS...]
   Generates a .syso file with specified resources embedded in .rsrc section,
   aimed for consumption by Go linker when building Win32 excecutables.
 
@@ -20,7 +20,7 @@ OPTIONS:
   -manifest string
     	path to a Windows manifest file to embed
   -o string
-    	name of output COFF (.res or .syso) file (default "rsrc.syso")
+    	name of output COFF (.res or .syso) file; if set to empty, will default to 'rsrc_windows_{arch}.syso'
 
 Based on ideas presented by Minux.
 

rsrc.go

@@ -10,7 +10,7 @@ import (
 
 var usage = `USAGE:
 
-%s [-manifest FILE.exe.manifest] [-ico FILE.ico[,FILE2.ico...]] -o FILE.syso
+%s [-manifest FILE.exe.manifest] [-ico FILE.ico[,FILE2.ico...]] [OPTIONS...]
   Generates a .syso file with specified resources embedded in .rsrc section,
   aimed for consumption by Go linker when building Win32 excecutables.
 
@@ -28,17 +28,20 @@ func main() {
 	flags := flag.NewFlagSet("", flag.ExitOnError)
 	flags.StringVar(&fnamein, "manifest", "", "path to a Windows manifest file to embed")
 	flags.StringVar(&fnameico, "ico", "", "comma-separated list of paths to .ico files to embed")
-	flags.StringVar(&fnameout, "o", "rsrc.syso", "name of output COFF (.res or .syso) file")
+	flags.StringVar(&fnameout, "o", "", "name of output COFF (.res or .syso) file; if set to empty, will default to 'rsrc_windows_{arch}.syso'")
 	flags.StringVar(&arch, "arch", "386", "architecture of output file - one of: 386, amd64, [EXPERIMENTAL: arm, arm64]")
 	flags.Usage = func() {
 		fmt.Fprintf(os.Stderr, usage, os.Args[0])
 		flags.PrintDefaults()
 	}
 	_ = flags.Parse(os.Args[1:])
-	if fnameout == "" || (fnamein == "" && fnameico == "") {
+	if fnamein == "" && fnameico == "" {
 		flags.Usage()
 		os.Exit(1)
 	}
+	if fnameout == "" {
+		fnameout = "rsrc_windows_" + arch + ".syso"
+	}
 
 	err := rsrc.Embed(fnameout, arch, fnamein, fnameico)
 	if err != nil {

rsrc_test.go

@@ -8,7 +8,7 @@ import (
 	"testing"
 )
 
-const name = "rsrc.syso"
+const name = "rsrc_windows_amd64.syso"
 
 func TestBuildSucceeds(t *testing.T) {
 	tests := []struct {
@@ -35,7 +35,7 @@ func TestBuildSucceeds(t *testing.T) {
 			// Compile icon/manifest in testdata/ dir
 			os.Stdout.Write([]byte("-- compiling resource(s)...\n"))
 			defer os.Remove(filepath.Join(dir, name))
-			cmd := exec.Command("go", "run", "../rsrc.go", "-arch", "amd64", "-o", name)
+			cmd := exec.Command("go", "run", "../rsrc.go", "-arch", "amd64")
 			cmd.Args = append(cmd.Args, tt.args...)
 			cmd.Dir = dir
 			cmd.Stdout = os.Stdout
@@ -45,6 +45,12 @@ func TestBuildSucceeds(t *testing.T) {
 				t.Fatal(err)
 			}
 
+			// Verify if a .syso file with default name was created
+			_, err = os.Stat(filepath.Join(dir, name))
+			if err != nil {
+				t.Fatal(err)
+			}
+
 			defer os.Setenv("GOOS", os.Getenv("GOOS"))
 			defer os.Setenv("GOARCH", os.Getenv("GOARCH"))
 			os.Setenv("GOOS", "windows")