Installing GMP-ECM on a Mac

This can either be done by compiling from the tarball in the normal way:
0) N.B. requirement for GMP library present first
1) untar
2) configure
3) make
4) (optionally) make install

or, more conveniently, download and install DarwinPorts, followed by simply:
1) 'sudo port install gmp-ecm'

[note the similarity to Ubuntu's apt-get process]