externals: Track upstream libusb

We can place the external in an inner folder and manage the custom files
necessary to integrate it with CMake directly. This allows us to
directly change how we use it with our build system, as opposed to
needing to change a fork.

externals/libusb/CMakeLists.txt

@@ -0,0 +1,147 @@
+add_library(usb STATIC EXCLUDE_FROM_ALL
+    libusb/libusb/core.c
+    libusb/libusb/core.c
+    libusb/libusb/descriptor.c
+    libusb/libusb/hotplug.c
+    libusb/libusb/io.c
+    libusb/libusb/strerror.c
+    libusb/libusb/sync.c
+)
+set_target_properties(usb PROPERTIES VERSION 1.0.23)
+if(WIN32)
+    target_include_directories(usb
+        BEFORE
+        PUBLIC
+          libusb/libusb
+
+        PRIVATE
+          "${CMAKE_CURRENT_BINARY_DIR}"
+    )
+
+    if (NOT MINGW)
+        target_include_directories(usb BEFORE PRIVATE libusb/msvc)
+    endif()
+else()
+target_include_directories(usb
+    # turns out other projects also have "config.h", so make sure the
+    # LibUSB one comes first
+    BEFORE
+
+    PUBLIC
+      libusb/libusb
+
+    PRIVATE
+      "${CMAKE_CURRENT_BINARY_DIR}"
+)
+endif()
+
+if(WIN32 OR CYGWIN)
+    target_sources(usb PRIVATE
+      libusb/libusb/os/threads_windows.c
+      libusb/libusb/os/windows_winusb.c
+      libusb/libusb/os/windows_usbdk.c
+      libusb/libusb/os/windows_nt_common.c
+    )
+    set(OS_WINDOWS TRUE)
+elseif(APPLE)
+    target_sources(usb PRIVATE
+        libusb/libusb/os/darwin_usb.c
+    )
+    find_library(COREFOUNDATION_LIBRARY CoreFoundation)
+    find_library(IOKIT_LIBRARY IOKit)
+    find_library(OBJC_LIBRARY objc)
+    target_link_libraries(usb PRIVATE
+        ${COREFOUNDATION_LIBRARY}
+        ${IOKIT_LIBRARY}
+        ${OBJC_LIBRARY}
+    )
+    set(OS_DARWIN TRUE)
+elseif(ANDROID)
+    target_sources(usb PRIVATE
+        libusb/libusb/os/linux_usbfs.c
+        libusb/libusb/os/linux_netlink.c
+    )
+    find_library(LOG_LIBRARY log)
+    target_link_libraries(usb PRIVATE ${LOG_LIBRARY})
+    set(OS_LINUX TRUE)
+elseif(${CMAKE_SYSTEM_NAME} MATCHES "Linux")
+    target_sources(usb PRIVATE
+        libusb/libusb/os/linux_usbfs.c
+    )
+    find_package(Libudev)
+    if(LIBUDEV_FOUND)
+        target_sources(usb PRIVATE
+            libusb/libusb/os/linux_udev.c
+        )
+        target_link_libraries(usb PRIVATE "${LIBUDEV_LIBRARIES}")
+        target_include_directories(usb PRIVATE "${LIBUDEV_INCLUDE_DIR}")
+        set(HAVE_LIBUDEV TRUE)
+        set(USE_UDEV TRUE)
+    else()
+        target_sources(usb PRIVATE
+            libusb/libusb/os/linux_netlink.c
+        )
+    endif()
+    set(OS_LINUX TRUE)
+elseif(${CMAKE_SYSTEM_NAME} MATCHES "NetBSD")
+    target_sources(usb PRIVATE
+        libusb/libusb/os/netbsd_usb.c
+    )
+    set(OS_NETBSD TRUE)
+elseif(${CMAKE_SYSTEM_NAME} MATCHES "OpenBSD")
+    target_sources(usb PRIVATE
+        libusb/libusb/os/openbsd_usb.c
+    )
+    set(OS_OPENBSD TRUE)
+endif()
+
+if(UNIX)
+    target_sources(usb PRIVATE
+        libusb/libusb/os/poll_posix.c
+        libusb/libusb/os/threads_posix.c
+    )
+    find_package(Threads REQUIRED)
+    if(THREADS_HAVE_PTHREAD_ARG)
+      target_compile_options(usb PUBLIC "-pthread")
+    endif()
+    if(CMAKE_THREAD_LIBS_INIT)
+      target_link_libraries(usb PRIVATE "${CMAKE_THREAD_LIBS_INIT}")
+    endif()
+    set(THREADS_POSIX TRUE)
+elseif(WIN32)
+    target_sources(usb PRIVATE
+        libusb/libusb/os/poll_windows.c
+        libusb/libusb/os/threads_windows.c
+    )
+endif()
+
+include(CheckFunctionExists)
+include(CheckIncludeFiles)
+include(CheckTypeSize)
+check_include_files(asm/types.h HAVE_ASM_TYPES_H)
+check_function_exists(gettimeofday HAVE_GETTIMEOFDAY)
+check_include_files(linux/filter.h HAVE_LINUX_FILTER_H)
+check_include_files(linux/netlink.h HAVE_LINUX_NETLINK_H)
+check_include_files(poll.h HAVE_POLL_H)
+check_include_files(signal.h HAVE_SIGNAL_H)
+check_include_files(strings.h HAVE_STRINGS_H)
+check_type_size("struct timespec" STRUCT_TIMESPEC)
+check_function_exists(syslog HAVE_SYSLOG_FUNC)
+check_include_files(syslog.h HAVE_SYSLOG_H)
+check_include_files(sys/socket.h HAVE_SYS_SOCKET_H)
+check_include_files(sys/time.h HAVE_SYS_TIME_H)
+check_include_files(sys/types.h HAVE_SYS_TYPES_H)
+
+set(CMAKE_EXTRA_INCLUDE_FILES poll.h)
+check_type_size("nfds_t" nfds_t)
+unset(CMAKE_EXTRA_INCLUDE_FILES)
+if(HAVE_NFDS_T)
+    set(POLL_NFDS_TYPE "nfds_t")
+else()
+    set(POLL_NFDS_TYPE "unsigned int")
+endif()
+
+check_include_files(sys/timerfd.h USBI_TIMERFD_AVAILABLE)
+
+
+configure_file(config.h.in config.h)