Makefile: use an include file for common variables

[thaJeztah]

With the assumption that;

• each makefile should be able to be used standalone (cd static && make <foo>)
• each makefile should also be able to be called from the top makefile (make static -> recurses over the makefiles)
• when called from the main makefile, common variables that are set in the main makefile should be (mostly) leading.
• it should be possible to manually override variables (make GO_VERSION=1.13beta1 static)

Tricky bits:

• paths ... because some Makefiles are in subdirectories, we cannot set (e.g.) CLI_DIR and ENGINE_DIR in the common.mk (because it's done relative to the working directory).
  • Perhaps some trickery could be done with realpath (docs says it returns an empty path if it's not found, so we could first try ../engine and if it's empty, try ../../engine)
• GO_VERSION; this is a bit unclear: why does static/Makefile detect the Go-version from the CLI, but other targets don't?
  • Should GO_VERSION be set to the CLI's version for all targets?
  • Or set to latest ?
  • Or empty (force a version to be set?)

STATIC_VERSION (see first commit)

I noticed that static version was always overridden (?), which meant that static packages would potentially get a different version than non-static versions if called from the main Makefile (make static)? (TBH; just from reading the code, haven't verified if they did)

[thaJeztah]

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