Allow disabling vendoring.

[baikiet]

psmoveapi ships its own version of PS3EYEDriver, glm, hidapi and libusb-1.0. It is often desirable for downstream maintainers to use their own version of these packages. However the psmoveapi CMake configuration currently does not allow that.

This change introduces a new PSMOVEAPI_VENDORING CMake variable which is enabled by default. When it is disabled, we disable all references to the external/ directory across the build system and we let CMake look for its dependencies in the usual locations.

This is still explicitly broken for Windows and Darwin as I have no access to such systems and am not able to test whether this works.

Successfully tested on Debian 12.13:

$ mkdir build && cd build && cmake -DPSMOVEAPI_VENDORING=OFF ..
-- The C compiler identification is GNU 12.2.0
-- The CXX compiler identification is GNU 12.2.0
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/bin/cc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Found PkgConfig: /usr/bin/pkg-config (found version "1.8.1")
-- Performing Test CMAKE_HAVE_LIBC_PTHREAD
-- Performing Test CMAKE_HAVE_LIBC_PTHREAD - Success
-- Found Threads: TRUE
-- Checking for module 'dbus-1'
--   Found dbus-1, version 1.14.10
-- Checking for module 'libudev'
--   Found libudev, version 252
-- Checking for module 'bluez>=5'
--   Found bluez, version 5.66
-- Checking for module 'hidapi-hidraw'
--   Found hidapi-hidraw, version 0.13.1
-- Checking for module 'glm'
--   Found glm, version 0.9.9.8

  Tracker
    Tracker library:       Yes (OpenCV version 4.6.0)
    Camera control driver: V4L2

  Build configuration
    Debug build:      No
    Library license:  BSD (see README.md for details)

  Additional targets
    C example apps:   Yes
    NavCon test:      Yes (using SDL2 joystick API)

-- Configuring done
-- Generating done
-- Build files have been written to: build
$ make
[  3%] Building C object CMakeFiles/psmoveapi.dir/src/psmove.c.o
[  6%] Building C object CMakeFiles/psmoveapi.dir/src/psmove_calibration.c.o
[  9%] Building CXX object CMakeFiles/psmoveapi.dir/src/psmove_orientation.cpp.o
[ 12%] Building CXX object CMakeFiles/psmoveapi.dir/src/psmoveapi.cpp.o
[ 16%] Building C object CMakeFiles/psmoveapi.dir/src/daemon/moved_monitor_linux.c.o
[ 19%] Building CXX object CMakeFiles/psmoveapi.dir/src/daemon/moved_client.cpp.o
[ 22%] Building CXX object CMakeFiles/psmoveapi.dir/src/platform/psmove_port_linux.cpp.o
[ 25%] Building CXX object CMakeFiles/psmoveapi.dir/src/math/psmove_alignment.cpp.o
[ 29%] Building C object CMakeFiles/psmoveapi.dir/src/math/psmove_math.c.o
[ 32%] Building CXX object CMakeFiles/psmoveapi.dir/src/math/psmove_quaternion.cpp.o
[ 35%] Building C object CMakeFiles/psmoveapi.dir/src/math/psmove_vector.c.o
[ 38%] Linking CXX shared library libpsmoveapi.so
[ 38%] Built target psmoveapi
[ 41%] Building CXX object CMakeFiles/psmoveapi_tracker.dir/src/tracker/psmove_tracker_hue_calibration.cpp.o
[ 45%] Building CXX object CMakeFiles/psmoveapi_tracker.dir/src/tracker/psmove_tracker.cpp.o
[ 48%] Building CXX object CMakeFiles/psmoveapi_tracker.dir/src/tracker/psmove_fusion.cpp.o
[ 51%] Building CXX object CMakeFiles/psmoveapi_tracker.dir/src/tracker/tracker_helpers.cpp.o
[ 54%] Building CXX object CMakeFiles/psmoveapi_tracker.dir/src/tracker/camera_control.cpp.o
[ 58%] Building CXX object CMakeFiles/psmoveapi_tracker.dir/src/tracker/camera_control_driver.cpp.o
[ 61%] Building CXX object CMakeFiles/psmoveapi_tracker.dir/src/tracker/camera_control_layouts.cpp.o
[ 64%] Building CXX object CMakeFiles/psmoveapi_tracker.dir/src/tracker/camera_control_driver_v4l2.cpp.o
[ 67%] Linking CXX shared library libpsmoveapi_tracker.so
[ 67%] Built target psmoveapi_tracker
[ 70%] Building CXX object CMakeFiles/psmove.dir/src/utils/psmovecli.cpp.o
[ 74%] Linking CXX executable psmove
[ 74%] Built target psmove
[ 77%] Building C object CMakeFiles/example.dir/examples/c/example.c.o
[ 80%] Linking C executable example
[ 80%] Built target example
[ 83%] Building C object CMakeFiles/multiple.dir/examples/c/multiple.c.o
[ 87%] Linking C executable multiple
[ 87%] Built target multiple
[ 90%] Building CXX object CMakeFiles/example_new_api.dir/examples/c/example_new_api.cpp.o
[ 93%] Linking CXX executable example_new_api
[ 93%] Built target example_new_api
[ 96%] Building CXX object CMakeFiles/test_navcon.dir/examples/c/test_navcon.cpp.o
[100%] Linking CXX executable test_navcon
[100%] Built target test_navcon

As per #510