Refactor parameter handling in build and sign scripts for PowerShell 5.1 compatibility

build-caffeine-exe.ps1

@@ -1,8 +1,8 @@
 [CmdletBinding()]
 param(
-    [string]$SourceScript = (Join-Path $PSScriptRoot 'caffeine.ps1'),
-    [string]$OutputDir = (Join-Path $PSScriptRoot 'dist'),
-    [string]$BuildDir = (Join-Path $PSScriptRoot 'build'),
+    [string]$SourceScript,
+    [string]$OutputDir,
+    [string]$BuildDir,
     [string]$ExeName = 'PortableCaffeine.exe',
     [switch]$ConsoleTarget,
     [switch]$KeepExtractedSource
@@ -11,6 +11,26 @@ param(
 Set-StrictMode -Version Latest
 $ErrorActionPreference = 'Stop'
 
+# In Windows PowerShell 5.1, $PSScriptRoot may be empty during param default evaluation.
+# Resolve script-relative defaults after the param block for compatibility.
+$scriptRoot = if (-not [string]::IsNullOrWhiteSpace($PSScriptRoot)) {
+    $PSScriptRoot
+} elseif ($MyInvocation.MyCommand.Path) {
+    Split-Path -Parent $MyInvocation.MyCommand.Path
+} else {
+    (Get-Location).Path
+}
+
+if ([string]::IsNullOrWhiteSpace($SourceScript)) {
+    $SourceScript = Join-Path $scriptRoot 'caffeine.ps1'
+}
+if ([string]::IsNullOrWhiteSpace($OutputDir)) {
+    $OutputDir = Join-Path $scriptRoot 'dist'
+}
+if ([string]::IsNullOrWhiteSpace($BuildDir)) {
+    $BuildDir = Join-Path $scriptRoot 'build'
+}
+
 function Get-CSharpFromCaffeineScript {
     param(
         [Parameter(Mandatory = $true)]