Fix hunspell dictionary symlink when built for flatpak
    
    We can't assume that the prefix where thunderbird is installed is the
    same where hunspell is: when building for flatpak, thunderbird is in
    /app, but hunspell dictionaries are in /usr.
    
    Fix this by asking hunspell pkg-config file where it is installed,
    instead of assuming it's in the same prefix as thunderbird.