CMAKE_INSTALL_PREFIX_INITIALIZED_TO_DEFAULT

CMake sets this variable to a TRUE value when the CMAKE_INSTALL_PREFIX has just been initialized to its default value, typically on the first run of CMake within a new build tree. This can be used by project code to change the default without overriding a user-provided value:

if(CMAKE_INSTALL_PREFIX_INITIALIZED_TO_DEFAULT)
  set(CMAKE_INSTALL_PREFIX "/my/default" CACHE PATH "..." FORCE)
endif()