Allow LCB build to use HdrHistogram_c from parent project

libcouchbase is part of Server builds. Both libcouchbase and Server builds use HdrHistogram_c. Since both Server and libcouchbase CMakeLists.txt do INCLUDE_SUBDIRECTORY() for HdrHistogram_c, the Server build fails at CMake time with a "duplicate target hdr_histogram" error.

For now we're working around this by setting LCB_USE_HDR_HISTOGRAM=off when building libcouchbase. This isn't ideal, though, as it disables HdrHistogram_c entirely for the libcouchbase build. It would be better for libcouchbase/CMakeLists.txt to detect that HdrHistogram_c is already available in the build (for instance by checking IF (TARGET hdr_histogram) ) and not adding the contrib/HdrHistogram_c directory in that case.