stack/slapos-py2.cfg: Pin gcc-8.5 only when scipy

stack/slapos-py2.cfg

@@ -6,7 +6,18 @@ part = python2.7
 
 [gcc]
 # To compile Python2-compatible scipy==1.0.1
-part = gcc-8.5
+depends += ${gcc-if-scipy:recipe}
+
+[gcc-if-scipy]
+recipe = slapos.recipe.build
+init =
+  # Note: doing "'scipy' in self.buildout" pulls [scipy]
+  # because Buildout inherits from DictMixin / MutableMapping
+  # making 'in' fall back to self.buildout.__getitem__('scipy').
+  # XXX: buildout should implement __contains__ directly.
+  if 'scipy' in self.buildout._raw:
+    # ok because [gcc]'s init is still unevaluated at this point
+    self.buildout['gcc']['part'] = 'gcc-8.5'
 
 [openssl]
 <= openssl-1.1