Fail configure if Alex 3 is found. Haddock doesn't build with Alex 3

yet, and fixing it is non-trivial (a change to Cabal is needed).  So
I'm making configure fail with an error suggesting to install an older
Alex for now.

configure.ac

@@ -608,6 +608,9 @@ dnl ** check for installed alex binary + version
 dnl    (don't do it if we're booting from .hc files though.)
 if test "$BootingFromHc" = "NO"; then
 FPTOOLS_ALEX
+if test "${Alex3}" = "YES"; then
+  AC_MSG_ERROR([GHC does not build with Alex 3 yet; please install an older version (we hope to fix this soon).])
+fi
 fi;
 
 dnl --------------------------------------------------