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TMPDIR is a special variable indicating the _system_ temporary directory root.
Unfortunately, go refuses to honor `go.mod` files in this directory to prevent
random `/tmp/go.mod` files from messing with modules cloned into `/tmp`.

The fix is simple: rename TMPDIR to TEMP.

The usage of a native 'go' command has been replaced with a make &
environment variable $GOCC. This enables building with multiple go
versions on a single machine as documented:
  * https://golang.org/doc/install#extra_versions

This enables the usage of:
```bash
$ make install
$ # OR
$ GOCC=go1.12.3 make install
$ # OR
$ GOCC=go1.12.4 make install
```
And the build and test tools now pick up on this change

 On branch go-version-check
 Changes to be committed:
	modified:   Rules.mk
	modified:   bin/check_go_version
	modified:   bin/dist_get
	modified:   bin/maketarball.sh
	modified:   coverage/Rules.mk
	modified:   mk/golang.mk
	modified:   mk/tarball.mk
License: MIT
Signed-off-by: Chris Buesser <christopher.buesser@gmail.com>

License: MIT
Signed-off-by: Jakub Sztandera <kubuxu@protonmail.ch>

Unfortunately, `readlink -f` doesn't work on macos. This change *manually*
converts a relative path to an absolute path.

fixes #5859

License: MIT
Signed-off-by: Steven Allen <steven@stebalien.com>

* don't write the tarball in the directory we're taring (makes tar unhappy)
* fix permissions
* fix gopath

License: MIT
Signed-off-by: Steven Allen <steven@stebalien.com>

Just stop using realpath, should not matter.

License: MIT
Signed-off-by: Jakub Sztandera <kubuxu@protonmail.ch>

License: MIT
Signed-off-by: Jakub Sztandera <kubuxu@protonmail.ch>