preventing 'make clean' from ExternalProject or add a custom clean with CMake

Question

My project has an external project that I add with its own CMake with the CMake ExternalProject_Add command.

Every time I add flags or do a rebuild the external project is also rebuilt and it takes a long time (rebuild of the external project includes extract and dynamic library build).

How do I prevent its rebuilding when we rebuild my project?

ExternalProject.cmake:

include(ExternalProject)

ExternalProject_Add(project
    PREFIX              ${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/external
    URL                 ${THIRD_PARTY_SRC}/archives/project.tar.gz
    INSTALL_DIR         ${THIRD_PARTY_SRC}/install
    CONFIGURE_COMMAND
    ${CMAKE_COMMAND}    -E env
    ${EXTERNAL_PROJECT_TOOLS}
    <SOURCE_DIR>/configure.py
    --prefix=<INSTALL_DIR>
    ${EXTERNAL_PROJECT_CPU}
    BUILD_COMMAND ${MAKE}
    )

CMakelists.txt:

set(PROJECT externalProject)
add_library(externalProject SHARED IMPORTED)
set_property(TARGET externalProject PROPERTY IMPORTED_LOCATION ${THIRD_PARTY_SRC}/install/lib/externalProject.so)