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

[NuLL3rr0r]

Well, I personally am in fabor of @bruce87en approach, for example libusb-cmake have these option names, which is clean:

set(LIBUSB_BUILD_SHARED_LIBS            OFF             CACHE BOOL "Build shared libraries for libusb")
set(LIBUSB_BUILD_TESTING                ${BUILD_TESTS}  CACHE BOOL "Build testing for libusb")
set(LIBUSB_BUILD_EXAMPLES               OFF             CACHE BOOL "Build examples for libusb")
set(LIBUSB_INSTALL_TARGETS              OFF             CACHE BOOL "Disable installation targets for libusb")
set(LIBUSB_TARGETS_INCLUDE_USING_SYSTEM OFF             CACHE BOOL "Use system includes for libusb")
set(LIBUSB_ENABLE_LOGGING               ON              CACHE BOOL "Enable logging in libusb")
set(LIBUSB_ENABLE_DEBUG_LOGGING         OFF             CACHE BOOL "Enable debug logging in libusb")
set(LIBUSB_ENABLE_UDEV                  ON              CACHE BOOL "Enable udev support in libusb")

add_subdirectory(third-party/libusb-cmake)

Where as:

set(BUILD_SHARED_LIBS       OFF     CACHE   BOOL        "Build as shared library")
set(ENABLE_EXAMPLES         FALSE   CACHE   INTERNAL    "True if you want to build the examples.")
set(ENABLE_TESTS            ${BUILD_TESTS}    CACHE   INTERNAL    "True if you want to build the tests.")
set(LIBUSBP_LOG             FALSE   CACHE   INTERNAL    "Output log messages to stderr for debugging.")
set(VBOX_LINUX_ON_WINDOWS   FALSE   CACHE   INTERNAL    "Skip tests known to cause problems on a Linux VirtualBox guest on Windows.")
set(ENABLE_GCOV             FALSE   CACHE   INTERNAL    "Compile with special options needed for gcov.")

add_subdirectory(third-party/libusbp)

unset(BUILD_SHARED_LIBS)
unset(ENABLE_EXAMPLES)
unset(ENABLE_TESTS)
#unset(LIBUSBP_LOG)
unset(VBOX_LINUX_ON_WINDOWS)
unset(ENABLE_GCOV)

The reason is we want to build some things statically and some things dynamically. His approach which is used by many cmake-based projects is more flexible, because does not impose an unset.