cmake: fix install error for shared libs

cmake/Modules/Packages/USER-SMD.cmake

@@ -18,7 +18,7 @@ if(DOWNLOAD_EIGEN3)
   set_target_properties(LAMMPS::EIGEN3 PROPERTIES INTERFACE_INCLUDE_DIRECTORIES "${SOURCE_DIR}")
   target_link_libraries(lammps PRIVATE LAMMPS::EIGEN3)
   add_dependencies(LAMMPS::EIGEN3 Eigen3_build)
-  if(BUILD_LIB)
+  if(BUILD_LIB AND NOT BUILD_SHARED_LIBS)
     install(CODE "MESSAGE(FATAL_ERROR \"Installing liblammps with downloaded libraries is currently not supported.\")")
   endif()
 else()