[ refactor for #265 ] doctests: don't rely on Library section in caba…

…l file

The include paths given to GHCI for running the doctests are more
stable by using dist/build/bnfc-doctests/autogen/ instead of
dist/build/autogen/ which is only present if BNFC is also a library.

source/test/doctests.hs

@@ -3,7 +3,7 @@ import Test.DocTest
 main = doctest
     [ "-icompat"
     , "-isrc"
-    , "-idist/build/autogen/"
+    , "-idist/build/bnfc-doctests/autogen/"
     , "-idist/build/bnfc/bnfc-tmp"
     , "-XLambdaCase"
     , "-XFlexibleContexts"