fix: don't squash -Wno-sign-compare when appending -Wno-register

I didn't realise that COMPILE_FLAGS is not an additive property and setting it
the way we were doing wipes out its previous value. In the case where the
compiler supports both these flags, the setting of -Wno-register was causing
-Wno-sign-compare to be lost and thus become re-enabled by the preceding -Wall.