[Gm2] much reduced building times for gm2 on gcc-5.2.0 branch

[Gaius Mulley]

Hello,

I've just git pushed some changes to gm2/Make-lang.in which
fix some cyclic dependencies causing long build times.  It also allows
^C to be hit and restarted from where it left off rather than rebuild
the parser generator which in turn triggered multiple rebuilds of
machine generated files

regards,
Gaius