Allow compilation with native GNU make and cmd only

[dra27]

These commits combine with some other work of mine which allow OCaml to be compiled using native GNU make (e.g. from ezwinports) and no Cygwin installation.

The bulk of the change consists of allowing Compat.ml to be generated without needing sed or test and requiring 2013's GNU make 4.0 to be able to use $(file ..) to generate version.ml without resorting to chains of echo commands.