[Mingw-cross-env-list] Add t4k_common library to mingw-cross-env?

[David Bruce]

Hi Volker (cc'd you in case the list disallows attachments),

Our t4k_common library for tuxmath and tuxtype has now shaped up to
the point where I would like to see it included in mingw-cross-env.
Basically, I'd like to see more devs in our group do the crossbuilds
themselves, and the fewer steps required, the better.

I've attached the t4k_common.mk I've been using.  It works, but it
doesn't support checking for updates.

Thanks for your consideration,

David Bruce

t4k_common.mk
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