[PATCH] BUILD: opentracing: display warning in case of using OT_USE_VARS at compile time

[Miroslav Zagorac]

Hello all,

to avoid misunderstandings (like say github issue #1493) when compiling
HAproxy source with the OpenTracing filter enabled, a warning has been
added if the OT_USE_VARS variable is used.

If appropriate, please apply this commit.

Best regards.

--
Miroslav Zagorac