BUILD: Makefile: disable -Wunused-label

It's becoming extremely tricky not to make gcc warn about unused labels
with support for openssl 1.1 and 1.1.1, because some error paths only exist
for certain versions. Latest patch causes a warning for me on 1.0.2. There
is no real point it warning about an unused error label so let's disable
this warning.
1 file changed