fix the configure script to work on Fedora 20

My echo does not interpret \n in the command by default, so the
following idiom fails and raises a CPP warning (extra stuff at the end
of an #include line):

  echo "#include <caml/mlvalues.h>\n SIZEOF_INT" | cpp -P

"echo -e" may be used to solve the problem, but Jun Inoue suggested
using printf instead for better portability.