macos is lying about the header's existence, so it doesn't get multi-…

…threading for particles

src/playsim/p_effect.cpp

@@ -34,7 +34,7 @@
 ** more useful.
 */
 
-#if __has_include(<execution>)
+#if __has_include(<execution>) && !__APPLE__
 
 	#include <execution>
 
@@ -63,7 +63,7 @@ CVAR (Int, r_rail_spiralsparsity, 1, CVAR_ARCHIVE);
 CVAR (Int, r_rail_trailsparsity, 1, CVAR_ARCHIVE);
 CVAR (Bool, r_particles, true, 0);
 
-#if __has_include(<execution>)
+#if __has_include(<execution>) && !__APPLE__
 
 	CVAR (Bool, r_particles_multithreaded, true, CVAR_ARCHIVE);
 
@@ -289,7 +289,7 @@ void P_ThinkParticles (FLevelLocals *Level)
 {
 	uint64_t startNs = I_nsTime();
 	std::deque<particle_t>::iterator newEnd;
-#if __has_include(<execution>)
+#if __has_include(<execution>) && !__APPLE__
 	if(r_particles_multithreaded)
 	{
 		newEnd = std::remove_if(std::execution::par_unseq, Level->Particles.begin(), Level->Particles.end(), [Level](particle_t &p)