Check the graphviz executable are there.

Even if the pydot Python package is installed, it is not certain that the graphviz executable are (that is the case with EPD for instance). In that case, we do not want to use it.

Check the graphviz executable are there.
7d544691 · Pascal Lamblin · 818bdf4b · 7d544691
--- a/theano/printing.py
+++ b/theano/printing.py
@@ -13,7 +13,10 @@ import numpy
 try:
    import pydot as pd
-    pydot_imported = True
+    if pd.find_graphviz():
+        pydot_imported = True
+    else:
+        pydot_imported = False
 except ImportError:
    pydot_imported = False