Building in debug and coverage when issuing the proper env variables

[filipecosta90]

Hi there @antirez , this PR enables building rax in debug and coverage modes by passing DEBUG=1 COV=1 environment variables.

The reason I opened this PR was that the DEBUG variable within the makefile is to generic and can lead to potential build issues on repos that use rax as a dependency. The following build command on a project ( named myproject ) that relies on rax would not generate a successfull rax build if the parent project uses DEBUG=1 as a normal way to build debug binaries.

project1: make DEBUG=1 myproject this would also trigger the build o rax that would fail due to DEBUG flags being now overriden by 1. Given that this is a common pratice (use DEBUG=1 env variable ) would you swith to this? wdyt?