make the userspace-rcu dependency optional

Disable the userpace-rcu support by default. Our used userspace-rcu flavor memb is rather new and not available in liburcu version 0.10 available in debian buster. This change switches from using DCE and "if constexpr" to the C preprocessor so the library is only needed when the userspace-rcu support is actually enabled.

Motivation is building the emper on our bigboxes.

emper/Runtime.cpp

@@ -8,7 +8,6 @@
 // Non portable.
 #include <sched.h>				// for cpu_set_t, CPU_SET, CPU_ZERO
 #include <sys/sysinfo.h>	// for get_nprocs
-#include <urcu.h>					// for rcu_register_thread
 
 #include <cstdlib>	// for rand, srand, abort
 #include <cstring>
@@ -23,9 +22,14 @@
 #include "Fiber.hpp"								 // for Fiber
 #include "RuntimeStrategy.hpp"			 // for RuntimeStrategy
 #include "RuntimeStrategyStats.hpp"	 // for RuntimeStrategyStats
+#include "emper-config.h"						 // IWYU pragma: keep
 #include "lib/DebugUtil.hpp"
 #include "strategies/ws/WsStrategy.hpp"	 // for WsStrategy, WsStrategy::INST...
 
+#ifdef EMPER_LIBURCU
+#include <urcu.h>	 // for rcu_register_thread
+#endif
+
 #ifndef NDEBUG
 #include <syscall.h>	// for SYS_gettid
 #include <unistd.h>		// for syscall
@@ -79,9 +83,9 @@ Runtime::Runtime(workerid_t workerCount, RuntimeStrategy& strategy, unsigned int
 		workerIds[i] = i;
 
 		auto thread_function = [](void* voidWorkerId) -> void* {
-			if constexpr (emper::LIBURCU) {
-				rcu_register_thread();
-			}
+#ifdef EMPER_LIBURCU
+			rcu_register_thread();
+#endif
 
 			return currentRuntime->workerLoop(voidWorkerId);
 		};