filesystem_view: do not link external packages

lib/spack/spack/filesystem_view.py

@@ -220,6 +220,11 @@ def add_extension(self, spec):
                       % spec.name)
             return False
 
+        if spec.external:
+            tty.warn(self._croot + 'Skipping external package: %s'
+                     % colorize_spec(spec))
+            return True
+
         try:
             if not spec.package.is_activated(self.extensions_layout):
                 spec.package.do_activate(
@@ -245,6 +250,11 @@ def add_standalone(self, spec):
                       % spec.name)
             return False
 
+        if spec.external:
+            tty.warn(self._croot + 'Skipping external package: %s'
+                     % colorize_spec(spec))
+            return True
+
         if self.check_added(spec):
             tty.warn(self._croot + 'Skipping already linked package: %s'
                      % colorize_spec(spec))