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

Please do not set the OT_USE_VARS configuration variable, as the source
will probably not be able to compile!  For now, this variable can only
be used for experimental purposes, and is not intended for wider use.

For further clarification, please see commit 4cb2c83f4.

Must be backported to 2.5.
1 file changed