The variable "ep_common_args" containes an argument CMAKE_INSTALL_PREFIX. for "normal" external projects (EP) defined in mitk.git the argument is set to <INSTALL_PATH>, which is correct. For EPs defined in extensions the argument "CMAKE_INSTALL_PREFIX" seems to be not set. This leads to some EPs failing because they expect an install prefix.
How to reproduce: Output "ep_common_args" in an EP cmake script like "Eigen.cmake" -> everything is fine/set. Output "ep_common_args" in an extension EP cmake script -> not set.