favor qsort over mergesort for stable release

src/lmptype.h

@@ -46,6 +46,13 @@
 #define PRId64 "ld"
 #endif
 
+// favor qsort over mergesort for stable release
+// TODO: to be removed after stable release
+
+#ifndef LMP_QSORT
+#define LMP_QSORT
+#endif
+
 namespace LAMMPS_NS {
 
 // enum used for KOKKOS host/device flags