ghc-head: propagate gmp and ncurses libraries

This is a bit of a hack. The real solution is to make GHC pass appropriate -L
flags when calling the linker to avoid errors such as this one:

    source root is mtl-2.1.3.1
    patching sources
    configuring
    [1 of 1] Compiling Main             ( Setup.hs, /tmp/nix-build-haskell-mtl-2.1.3.1.drv-0/Main.o )
    Linking Setup ...
    /nix/store/b8qhjrwf8sf9ggkjxqqav7f1m6w83bh0-binutils-2.23.1/bin/ld: cannot find -lgmp
    collect2: error: ld returned 1 exit status