Use managed GNUstep root for theme builds

Author: danjboyd

packaging/scripts/ensure-windows-theme-inputs.ps1

@@ -1,7 +1,15 @@
 [CmdletBinding()]
 param(
   [string]$ThemeWorkspace,
-  [string]$MsysRoot = $(if (-not [string]::IsNullOrWhiteSpace($env:MSYS2_LOCATION)) { $env:MSYS2_LOCATION } else { "C:\msys64" })
+  [string]$MsysRoot = $(if (-not [string]::IsNullOrWhiteSpace($env:GP_GNUSTEP_CLI_ROOT)) {
+      $env:GP_GNUSTEP_CLI_ROOT
+    } elseif (-not [string]::IsNullOrWhiteSpace($env:GNUSTEP_CLI_ROOT)) {
+      $env:GNUSTEP_CLI_ROOT
+    } elseif (-not [string]::IsNullOrWhiteSpace($env:MSYS2_LOCATION)) {
+      $env:MSYS2_LOCATION
+    } else {
+      "C:\msys64"
+    })
 )
 
 Set-StrictMode -Version Latest