BUILD: makefile: ensure that all USE_* handlers appear before CFLAGS are used

BUILD: makefile: ensure that all USE_* handlers appear before CFLAGS are used

It happens that a few "if USE_foo" were placed too low in the makefile,
and would mostly work by luck thanks to not using variables that were
already referenced before. The opentracing include is even trickier
because it extends OPTIONS_CFLAGS that was last read a few lines before
being included, but it only works because COPTS is defined as a macro and
not a variable, so it will be evaluated later. At least now it doesn't
touch OPTIONS_* anymore and since it's cleanly arranged, it will work by
default via the flags collector.

Let's just move these late USE_* handlers upper and place a visible
delimiter after them reminding not to add any after.