Fix compile issue when used as submodule

[bruce87en]

Fix compile issue when used as submodule:
1.Modify BUILD_SHARED_LIBS to BUILD_LIBUSBP_SHARED_LIBS to avoid potential conflicts.
2.Modify the header file include path relative to the current CMakeLists.txt to avoid compilation errors caused by the top-level project not being libusbp.

[DavidEGrayson]

Change 2 seems OK, but for change 1, the intention is to just use this standard CMake variable which automatically controls what type of library gets built, instead of defining our own:

https://cmake.org/cmake/help/latest/variable/BUILD_SHARED_LIBS.html

So I think the BUILD_SHARED_LIBS code should stay is it is, but you could elaborate on what specific problem it's causing for you