Makefile: fix "no such file" error on first invocation

[benesch]

Currently, if you run Make on a fresh Git clone without a .buildinfo directory, you'll see the following output printed

$ make
cat: .buildinfo/tag: No such file or directory
cat: .buildinfo/rev: No such file or directory
cat: .buildinfo/tag: No such file or directory
cat: .buildinfo/rev: No such file or directory
cat: .buildinfo/rev: No such file or directory

after which Make, remarkably, manages to proceed with a successful build. (You can see this in an existing clone with rm -rf .buildinfo && make.) As best as I can tell, this is because Make treats LDFLAGS specially and expands it before executing any target recipe, even if that recipe does not reference LDFLAGS. Go figure.

Our current Makefile assumes the LDFLAGS variable won't be expanded until it's referenced in the install recipe, at which point .buildinfo/tag and .buildinfo/rev are guaranteed to exist, because they're prerequisites of install. But due to the above oddity, LDFLAGS is expanded during execution the recipes to create .buildinfo/*, and since LDFLAGS expands to $(shell cat .buildinfo/...), we see the above error message--but only on the first invocation, when .buildinfo/* does not yet exist. Note that the build proceeds successfully because LDFLAGS is re-expanded when executing the install target, where .buildinfo/* exists with the correct values.

This commit works around the issue by using a variable which behaves normally (namely, GOLDFLAGS), in place of LDFLAGS.

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NOTE: I'm perfectly happy to pursue another approach here (or at least drop an explanatory comment in the Makefile if this approach seems reasonable). For example, we could instead set LDFLAGS as a shell variable inside of the install recipe (rather than as a target-specific variable). The downside with that approach is that we'd lose the output of the expanded command: instead of go build... -ldflags '-X ...build.tag=12345', we'd see go build... -ldflags "$ldflags".

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This change is 

[bdarnell]

LGTM. I was just thinking the other day that it was odd that we were using the LDFLAGS variable here when that's also used (with different semantics) by make's default implicit rules (which we fortunately don't use).

One nit: Maybe call it GO_LDFLAGS (or LINKFLAGS?) to avoid confusion with https://en.wikipedia.org/wiki/Gold_(linker)

[benesch]

TFTRs! I went with LINKFLAGS because I think it's a bit less confusing name and added a comment with a few lines of rationale as well.

[tamird]

can this be up near the other flags? this one may conceivably be overridden on the command line or in the environment. for now, we can put it in the command-line only section.