scons: Monkey patch a fix from 3.0.2 into 3.0.0 and 3.0.1.

Ensure that SCons.Subst.Literal can be hashed.

https: //pairlist2.pair.net/pipermail/scons-dev/2018-October/004766.html
Change-Id: I5221b63a5fd63189eb690c77e780805dc6d68125
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/58633
Maintainer: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>

SConstruct

@@ -94,6 +94,14 @@ import SCons.Node
 import SCons.Node.FS
 import SCons.Tool
 
+if getattr(SCons, '__version__', None) in ('3.0.0', '3.0.1'):
+    # Monkey patch a fix which appears in version 3.0.2, since we only
+    # require version 3.0.0
+    def __hash__(self):
+        return hash(self.lstr)
+    import SCons.Subst
+    SCons.Subst.Literal.__hash__ = __hash__
+
 
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