Dmitry Vyukov d1a8851085 sys/targest: introduce target.BuildOS ...

We can't cross-compile native binaries from just any OS to any other.
For most OSes we can do only native compilation.
Some can only be compiled from linux.
To date we avoided this problem completely (mostly assumed linux build OS).
Make this notion of what can build what explicit.

2018-11-17 11:42:22 -08:00

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env.go

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