qga/wixl: simplify some pre-processing

Sadly, wixl doesn't have 'elif'.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Konstantin Kostiuk <kkostiuk@redhat.com>
Message-Id: <20220525144140.591926-14-marcandre.lureau@redhat.com>

qga/installer/qemu-ga.wxs

@@ -1,21 +1,15 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <Wix xmlns="http://schemas.microsoft.com/wix/2006/wi">
-  <?ifndef var.Arch?>
-    <?error Define Arch to 32 or 64?>
-  <?endif?>
-
   <?if $(var.Arch) = "64"?>
     <?define ArchLib=libgcc_s_seh-1.dll?>
     <?define GaProgramFilesFolder="ProgramFiles64Folder" ?>
-  <?endif?>
-
-  <?if $(var.Arch) = "32"?>
-    <?define ArchLib=libgcc_s_dw2-1.dll?>
-    <?define GaProgramFilesFolder="ProgramFilesFolder" ?>
-  <?endif?>
-
-  <?ifndef var.ArchLib ?>
-    <?error Unexpected Arch value $(var.Arch)?>
+  <?else?>
+    <?if $(var.Arch) = "32"?>
+      <?define ArchLib=libgcc_s_dw2-1.dll?>
+      <?define GaProgramFilesFolder="ProgramFilesFolder" ?>
+    <?else?>
+      <?error Unexpected Arch value $(var.Arch)?>
+    <?endif?>
   <?endif?>
 
   <Product