Support setting version string via ldflags

When xcaddy is built directly from source (e.g. distro packaging),
Go's debug.ReadBuildInfo() reports the main module version as
"(devel)" rather than the actual release version. The upstream
build process works around this by building from a separate wrapper
module that imports xcaddy as a versioned dependency, but packagers
typically build directly from the source tarball.

Add a version variable that can be set via -ldflags -X at build
time, allowing packagers to inject the correct version string.

Author: p5

cmd/main.go

@@ -33,6 +33,11 @@ import (
 	"github.com/caddyserver/xcaddy/internal/utils"
 )
 
+// version can be set at build time via -ldflags to override the version
+// reported by debug.ReadBuildInfo() (which returns "(devel)" when built
+// as the main module, e.g. in distro packaging).
+var version string
+
 var (
 	caddyVersion     = os.Getenv("CADDY_VERSION")
 	raceDetector     = os.Getenv("XCADDY_RACE_DETECTOR") == "1"
@@ -207,6 +212,9 @@ func splitWith(arg string) (module, version, replace string, err error) {
 
 // xcaddyVersion returns a detailed version string, if available.
 func xcaddyVersion() string {
+	if version != "" {
+		return version
+	}
 	mod := goModule()
 	ver := mod.Version
 	if mod.Sum != "" {