add install-python target with suitable error message also when not building a shared library

cmake/CMakeLists.txt

@@ -1488,6 +1488,10 @@ if(BUILD_LIB AND BUILD_SHARED_LIBS)
     WORKING_DIRECTORY  ${CMAKE_CURRENT_SOURCE_DIR}/../python
     COMMENT "Installing LAMMPS Python module"
     )
+else()
+  add_custom_target(
+    install-python
+    echo "Installation of the LAMMPS Python module requires building the LAMMPS shared library")
 endif()
 
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