alex 3 incompatibility workaround

As a temporary workaround, alex is no longer invoked automatically when
building with cabal. Developers who want to modify the lexer need to run
alex on Lexer.x manually and record the modified Lexer.hs.

    src/compiler/GF/Grammar/lexer/Lexer.x    -- hidden from cabal
    src/compiler/GF/Grammar/Lexer.hs         -- update it manually

gf.cabal

@@ -111,7 +111,8 @@ executable gf
     other-modules: GFServer
     hs-source-dirs: src/server src/server/transfer src/example-based
 
-  build-tools: happy, alex>=2 && <3
+  build-tools: happy
+           --, alex>=2 && <3 -- tricky to install in Ubuntu 12.04
   if os(windows)
     build-depends: Win32
   else