compiling: Add -i flag to go build to speed up builds

So builds take about 30s on my shitty machine which is pretty long.
Turns out golang caches things in $GOPATH/pkg/ but is not clever enough
to erase things from there that are out of date. As a result, it was
rebuilding everything (including the unchanged dependencies) every
build!

By completely wiping out $GOPATH/pkg/ and then running `go build -i`,
this now takes builds down to about 8 seconds. (After one full build is
finished.)

This is basically the same as running `go install`, but without copying
junk to $GOPATH/bin.

Hopefully the tooling will be smart enough to know when to throw out
stuff in $GOPATH/pkg automatically and avoid this problem entirely.

Is it wrong to send Google a bill for all the extra cpu cycles I've
used? ;)

Makefile

@@ -112,7 +112,7 @@ ifneq ($(OLDGOLANG),)
 	@# avoid equals sign in old golang versions eg in: -X foo=bar
 	time go build -ldflags "-X main.program $(PROGRAM) -X main.version $(SVERSION)" -o $(PROGRAM) $(BUILD_FLAGS);
 else
-	time go build -ldflags "-X main.program=$(PROGRAM) -X main.version=$(SVERSION)" -o $(PROGRAM) $(BUILD_FLAGS);
+	time go build -i -ldflags "-X main.program=$(PROGRAM) -X main.version=$(SVERSION)" -o $(PROGRAM) $(BUILD_FLAGS);
 endif
 
 $(PROGRAM).static: main.go