fix infinite fork loop when dep fails in 'make vendor-install'

[nuno-silva]

In #164, a || $(MAKE) vendor-install was added to the vendor-install target to help with "poor network circumstances":

gluster-prometheus/Makefile

Lines 82 to 85 in 3ebaacc

	vendor-install:
	@echo Installing vendored packages
	@$(DEPENV) dep ensure -v -vendor-only || $(MAKE) vendor-install
	@echo

However, this leads to an infinite fork loop when dep fails or is not installed, possibly crashing the host machine.

This PR works around the infinite loop by checking the MAKELEVEL and aborting if it gets too high, which still allows the command to be retried a reasonable number of times.
Another possible approach is just looping around dep a set number of times.

Fixes #195.