Makefile: export PWD for OpenBSD's ksh(1)

Interestingly, ksh(1) on OpenBSD does not export PWD by default, and it
also has a notion of the "logical cwd" vs the "physical cwd", with the
latter being passed to chdir, but the former being stored in the
non-exported PWD and displayed to the user. This means that if you `cd`
into a directory that's comprised of symlinks, exec'd processes will see
the physical path. Observe:

  # ksh
  # mkdir a
  # ln -s a b
  # cd b
  # pwd
  /root/b
  # ksh -c pwd
  /root/a

The fact of separating physical and logical paths is not too uncommon
for shells (bash does it too), but not exporting PWD is very odd.

Since this is common behavior for many shells, libraries that return the
working directory will do something strange: they `stat(".")` and then
`stat(getenv("PWD"))`, and if these point to the same inode, they roll
with the value of `getenv("PWD")`, or otherwise fallback to asking the
kernel for the cwd.

Since PWD was not exported by ksh(1), Go's dep utility did not understand
it was operating inside of our faked GOPATH and became upset.

This patch works around the whole situation by simply exporting PWD
before executing dep.

Makefile

@@ -30,7 +30,7 @@ version.go:
 
 vendor/.created: Gopkg.toml Gopkg.lock | .gopath/.created
 	command -v dep >/dev/null || go get -v github.com/golang/dep/cmd/dep
-	cd .gopath/src/$(GO_IMPORT_PATH) && dep ensure -vendor-only -v
+	export PWD; cd .gopath/src/$(GO_IMPORT_PATH) && dep ensure -vendor-only -v
 	touch $@
 
 wireguard-go: $(wildcard *.go) $(wildcard */*.go) .gopath/.created vendor/.created version.go