Oops, fix compilation error on Linux & Windows

makepanda/makepanda.py

@@ -3302,7 +3302,8 @@ if (PkgSkip("PYTHON")==0):
   TargetAdd('libp3direct.dll', input='dcparser_dcLexer.obj')
   TargetAdd('libp3direct.dll', input='libdcparser_igate.obj')
   TargetAdd('libp3direct.dll', input='showbase_showBase.obj')
-  TargetAdd('libp3direct.dll', input='showbase_showBase_assist.obj')
+  if (sys.platform == "darwin"):
+    TargetAdd('libp3direct.dll', input='showbase_showBase_assist.obj')
   TargetAdd('libp3direct.dll', input='libshowbase_igate.obj')
   TargetAdd('libp3direct.dll', input='deadrec_composite.obj')
   TargetAdd('libp3direct.dll', input='libdeadrec_igate.obj')