BUILD: makefile: support USE_xxx=0 as well

BUILD: makefile: support USE_xxx=0 as well

William rightfully reported that not supporting =0 to disable a USE_xxx
option is sometimes painful (e.g. a script might do USE_xxx=$(command)).
It's not that difficult to handle actually, we just need to consider the
value 0 as empty at the few places that test for an empty string in
options.mk, and in each "ifneq" test in the main Makefile, so let's do
that. We even take care of preserving the original value in the build
options string so that building with USE_OPENSSL=0 will be reported
as-is in haproxy -vv, and with "-OPENSSL" in the feature list.