After commit b14c1a3a9c , packages could not
be installed on Debian <= Wheezy, because it lacks package
libtool-bin. So package mxe-requirements should depend on
libtool-bin only for Debian >= Jessie.
This commit changes the function that builds packages
mxe-requirements. Now it makes 4 .deb files:
wheezy/
mxe-requirements_2.23_i386.deb
mxe-requirements_2.23_amd64.deb
jessie/
mxe-requirements_2.23_i386.deb
mxe-requirements_2.23_amd64.deb
see #754