restore old qsort() based code and add preprocessor directives to switch

-DLMP_USE_LIBC_QSORT will use qsort() from libc to sort (requires static/global variables).
-DLMP_USE_MERGE_SORT will use a plain merge sort. slightly slower for expensive comparisons.
-DLMP_USE_HYBRID_SORT will use hybrid merge sort. faster than merge sort (no static/global variables)

src/irregular.h

@@ -21,6 +21,12 @@ namespace LAMMPS_NS {
 class Irregular : protected Pointers {
  public:
 
+#if defined(LMP_USE_LIBC_QSORT)
+  // static variable across all Irregular objects, for qsort callback
+
+  static int *proc_recv_copy;
+#endif
+
   Irregular(class LAMMPS *);
   ~Irregular();
   void migrate_atoms(int sortflag = 0, int preassign = 0,