try another variant

src/Makefile.package.empty

@@ -7,6 +7,6 @@ PKG_LIB =
 PKG_CPP_DEPENDS = 
 PKG_LINK_DEPENDS = 
 
-PKG_SYSINC = -I../../third_party
+PKG_SYSINC = -I../../third_party -I../third_party
 PKG_SYSLIB = 
 PKG_SYSPATH = 

src/json.h

@@ -17,7 +17,7 @@
 // wrapper around including the JSON parsing and writing class
 // Do NOT include in any header file
 
-#include <nlohmann/json.hpp>
+#include "nlohmann/json.hpp"
 
 namespace LAMMPS_NS {
 using json = ::nlohmann_lmp::json;