Fixed bug in Spec._dup, updated docstring

The private method `Spec._dup` was missing a line (when setting compiler
flags the parent spec was not set to `self`). This resulted in
an inconsistent state of the duplicated Spec. This problem has been
fixed here. The docstring of `Spec._dup` has been updated.

lib/spack/spack/spec.py

@@ -2548,10 +2548,19 @@ def _dup(self, other, deps=True, cleardeps=True):
 
         To duplicate an entire DAG, call _dup() on the root of the DAG.
 
-           Options:
-           dependencies[=True]
-               Whether deps should be copied too.  Set to False to copy a
-               spec but not its dependencies.
+        Args:
+            other (Spec): spec to be copied onto ``self``
+            deps (bool or Sequence): if True copies all the dependencies. If
+                False copies None. If a sequence of dependency types copy
+                only those types.
+            cleardeps (bool): if True clears the dependencies of ``self``,
+                before possibly copying the dependencies of ``other`` onto
+                ``self``
+
+        Returns:
+            True if ``self`` changed because of the copy operation,
+            False otherwise.
+
         """
         # We don't count dependencies as changes here
         changed = True
@@ -2577,6 +2586,7 @@ def _dup(self, other, deps=True, cleardeps=True):
             self._dependents = DependencyMap(self)
             self._dependencies = DependencyMap(self)
         self.compiler_flags = other.compiler_flags.copy()
+        self.compiler_flags.spec = self
         self.variants = other.variants.copy()
         self.variants.spec = self
         self.external_path = other.external_path