for distributions on systems where "polymake" exists, make Macaulay2 depend on it

how to distinguish the others and clarify to the user the need to download it?

if these problems with it are solved, we could include it:
    it uses a dynamic readline library that can only be provided by fink and probably will not be copied into the install location
    it offers no way to provide a directory tree containing a readline library we've compiled
    it writes a file into the user's home directory called ".polymake"
    it asks questions

Nov 8, 2010: Michael Joswig reports they've addressed those problems by reworking the build process

work around the problem of leaving stuff in the home directory by setting
POLYMAKE_USER_DIR to ".", or to a subdirectory of the build/source directory

the version in fink is out of date, get the tarball and use that
