Enable eager blackholing

While usually this is merely an optimization in parallel settings, in
our case it's absolutely essential for correctness due to the use of
internal mutability. See #147.

Author: bgamari

unordered-containers.cabal

@@ -50,6 +50,11 @@ library
   ghc-options: -Wall -O2
   if impl(ghc >= 6.8)
     ghc-options: -fwarn-tabs
+  if impl(ghc >= 6.12)
+    -- This is absolutely essential for correctness due to the
+    -- referential-transparency-breaking mutability in unsafeInsertWith. See
+    -- #147 and GHC #13615 for details.
+    ghc-options: -feager-blackholing
   if flag(debug)
     cpp-options: -DASSERTS