OpenMP required by libperf.c's ucx_perf_do_warmup

[jakirkham]

Most code in UCX that uses OpenMP is guarded to check that OpenMP is enabled. If it is not, there is fallback logic that doesn't use OpenMP

For example this happens in libperf_thread.c where an OpenMP block is provided if OpenMP is found

ucx/src/tools/perf/lib/libperf_thread.c

Lines 21 to 25 in 3c6b20d

	#if _OPENMP
	# include <omp.h>
	static ucs_status_t ucx_perf_thread_run_test(void* arg)

Otherwise a non-OpenMP fallback is provided

ucx/src/tools/perf/lib/libperf_thread.c

Lines 138 to 147 in 3c6b20d

	#else
	ucs_status_t ucx_perf_thread_spawn(ucx_perf_context_t *perf,
	ucx_perf_result_t* result)
	{
	ucs_error("Invalid test parameter (thread mode requested without OpenMP capabilities)");
	return UCS_ERR_INVALID_PARAM;
	}
	#endif /* _OPENMP */

As a result any #pragma omp lines live in the OpenMP enabled block

ucx/src/tools/perf/lib/libperf_thread.c

Line 44 in 3c6b20d

#pragma omp barrier

So are not encountered outside it

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With libperf.c however the OpenMP #include is optional

ucx/src/tools/perf/lib/libperf.c

Lines 26 to 28 in 3c6b20d

	#if _OPENMP
	# include <omp.h>
	#endif /* _OPENMP */

Though later in the code #pragma omp is not guarded

ucx/src/tools/perf/lib/libperf.c

Lines 1702 to 1706 in 3c6b20d

	#pragma omp barrier
	#pragma omp single copyprivate(status)
	/* Synchronize on whether to continue or stop the warmup phase */
	status = ucx_perf_test_exchange_status(perf, stop_status);
	#pragma omp barrier

As a result building without OpenMP leads to errors like this

make[3]: Entering directory '/home/conda/feedstock_root/build_artifacts/ucx-split_1714181624237/work/src/tools/perf/lib'
  CC       libucxperf_la-libperf.lo
  CC       libucxperf_la-libperf_memory.lo
libperf.c: In function 'ucx_perf_do_warmup':
libperf.c:1356: error: ignoring '#pragma omp barrier' [-Werror=unknown-pragmas]
 1356 | #pragma omp barrier
      | 
libperf.c:1357: error: ignoring '#pragma omp single' [-Werror=unknown-pragmas]
 1357 | #pragma omp single copyprivate(status)
      | 
libperf.c:1360: error: ignoring '#pragma omp barrier' [-Werror=unknown-pragmas]
 1360 | #pragma omp barrier
      | 

This comes from a UCX 1.16.0 build. Here is the full build log

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Would it be possible to guard the OpenMP code above and provide a fallback to allow building without OpenMP?