In case the OpenTracing C Wrapper library was installed as part of the
system (ie in a directory that pkg-config considers part of the system),
HAProxy building was not possible because in that case pkg-config did
not set the value of the OT_CFLAGS variable in the addon Makefile.
This resolves GitHub issue #1323.
This one is the last optional module to build with haproxy, so let's move
it to addons/. It was renamed to "ot" as it was the only one whose USE_*
option did not match the directory name, now this is consistent.
Few changes were required, only the Makefile, and doc were adjusted, as
the directory was already self-contained and relocatable.