This wrapper script *should* enable GHC 7.* to work on systems with Xcode 5. To use it, drop this script somewhere, make it executable, and run.... Then follow the instructions it prints out. What it will do is, instruction you to put a copy in /usr/bin, then re-run it sudo. It will then find all your GHC 7 settings files, and patch them to make…

ghc-clang-wrapper

#!/bin/sh

inPreprocessorMode () {
    hasE=0
    hasU=0
    hasT=0
    for arg in "$@"
    do
        if [ 'x-E' = "x$arg" ];             then hasE=1; fi
        if [ 'x-undef' = "x$arg" ];         then hasU=1; fi
        if [ 'x-traditional' = "x$arg" ];   then hasT=1; fi
    done
    [ "$hasE$hasU$hasT" = '111' ]
}

gccIsClang () {
    gcc --version 2>/dev/null | grep -q clang
}

extraClangArgs="-Wno-invalid-pp-token -Wno-unicode -Wno-trigraphs"

adjustPreprocessorLanguage () {
    newArgs=''
    while [ $# -gt 0 ]
    do
        newArgs="$newArgs $1"
        if [ "$1" = '-x' ]
        then
            shift
            if [ $# -gt 0 ]
            then
                if [ "$1" = 'c' ]
                then
                    newArgs="$newArgs assembler-with-cpp"
                else
                    newArgs="$newArgs $1"
                fi
            fi
        fi
        shift
    done
    echo $newArgs
}

installThisScript () {
    if [ \! -x /usr/bin/ghc-clang-wrapper ]
    then
        echo "Copy this execute script to /usr/bin/ghc-clang-wrapper, then re-run it."
        echo "You can copy it with these commands:"
        echo
        echo sudo cp $0 /usr/bin/ghc-clang-wrapper
        echo sudo chmod 755 /usr/bin/ghc-clang-wrapper
        echo sudo ghc-clang-wrapper
        exit 1
    fi

    settings=`ls -1 /Library/Frameworks/GHC.framework/Versions/7*/usr/lib/ghc-7*/settings 2>/dev/null`

    if [ "$settings" = '' ]
    then
        echo "You don't seem to have Haskell Platform installed."
        echo "Visit http://www.haskell.org/platform/ to get and install it."
        echo "Then run this script again."
        echo
        echo "If you have GHC installed in a non-standard place, find the file"
        echo "named 'settings', that is alongside the compiler, and edit the"
        echo "c compiler line to read:"
        echo '     ("C compiler command", "/usr/bin/ghc-clang-wrapper"),'
        exit 1
    fi

    anyNeedPatching=0
    for sfile in $settings
    do
        if grep -q ghc-clang-wrapper $sfile
        then
            echo ALREADY PATCHED: $sfile
        else
            if [ `id -u` -eq 0 ]
            then
                sed -e '/C compiler command/s:"[^ ]*gcc":"/usr/bin/ghc-clang-wrapper":' -i '.bak' $sfile
                echo PATCHED: $sfile
            else
                echo WOULD PATCH: $sfile
                anyNeedPatching=1
            fi
        fi
    done

    if [ $anyNeedPatching -eq 1 ]
    then
        echo "Please run this script sudo to actually patch GHC:"
        echo sudo $0
    fi
}

if [ $# -eq 0 ]
then
    installThisScript
else
    if gccIsClang
    then
        if inPreprocessorMode "$@"
        then
            exec gcc $extraClangArgs `adjustPreprocessorLanguage "$@"`
        else
            exec gcc $extraClangArgs "$@"
        fi
    else
        exec gcc "$@"
    fi
fi

I've fixed the -nodefaultlibs warning in https://gist.github.com/tibbe/8448715.

Works beautifully, thank you!

Solved my problems after recently migrating to OSX. Thanks.

Works nicely, thanks!

Is this still relevant on OS X Yosemite?