How to make a subproject to be inside a project?

Code
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Hello. Let’s imagine a situation - there is a main project MainPrj and a subroject SubPrj, which is fetched as git submodule. SubPrj is self-sufficient cmake project with its own install target. MainPrj is mine, SubPrj is someone else’s, i can not modify it. If i add SubPrj to MainPrj via add_subdirectory(), then the install target will populate a destination as if the projects are independent of each other, each in its own directory. I would like SubPrj to be installed under some MainPrj install directory. For exapmle - desired install layout:

usr/
└── include
    └── MainPrj
        ├── main.h
        └── SubPrj
            └── sub.h

I.e, I would like to somehow set the prefix for SubPrj, so that SubPrj is used exclusive by MainPrj and does not create any name collisions in the global space. Do I have any options besides installing in /opt or rewriting CMakeLists.txt for SubPrj? I have considered ExternalProject_add() and execut_process(), but I don’t think that it is what i need . Thank you in advance.

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If SubPj doesn’t allow for manipulation of its install prefixes through variables (such as GNUInstallDirs) or other means, you’ll have to patch it. Said another way, if a project just says DESTINATION "include/me" literally, there’s no override available for it.