cmake: unbreak system libusb support

Reference libusb doesn't support DragonFly and FreeBSD because those
ship a different libusb implementation (supports 0.1, 1.0, 2.0 API).

ld: error: undefined symbol: usbi_backend
>>> referenced by core.c
>>>               core.c.o:(usbi_alloc_device) in archive externals/libusb/libusb.a

ld: error: undefined symbol: usbi_backend
>>> referenced by core.c
>>>               core.c.o:(usbi_alloc_device) in archive externals/libusb/libusb.a

ld: error: undefined symbol: usbi_backend
>>> referenced by core.c
>>>               core.c.o:(libusb_has_capability) in archive externals/libusb/libusb.a

ld: error: undefined symbol: usbi_backend
>>> referenced by core.c
>>>               core.c.o:(libusb_has_capability) in archive externals/libusb/libusb.a

ld: error: undefined symbol: usbi_backend
>>> referenced by core.c
>>>               core.c.o:(libusb_has_capability) in archive externals/libusb/libusb.a

ld: error: undefined symbol: usbi_backend
>>> referenced by core.c
>>>               core.c.o:(usbi_connect_device) in archive externals/libusb/libusb.a

ld: error: undefined symbol: usbi_backend
>>> referenced by core.c
>>>               core.c.o:(usbi_disconnect_device) in archive externals/libusb/libusb.a

ld: error: undefined symbol: usbi_backend
>>> referenced by core.c
>>>               core.c.o:(libusb_get_device_list) in archive externals/libusb/libusb.a

ld: error: undefined symbol: usbi_backend
>>> referenced by core.c
>>>               core.c.o:(libusb_get_device_list) in archive externals/libusb/libusb.a

ld: error: undefined symbol: usbi_backend
>>> referenced by core.c
>>>               core.c.o:(libusb_unref_device) in archive externals/libusb/libusb.a

ld: error: undefined symbol: usbi_backend
>>> referenced by core.c
>>>               core.c.o:(libusb_unref_device) in archive externals/libusb/libusb.a

ld: error: undefined symbol: usbi_write
>>> referenced by core.c
>>>               core.c.o:(usbi_signal_event) in archive externals/libusb/libusb.a

ld: error: undefined symbol: usbi_read
>>> referenced by core.c
>>>               core.c.o:(usbi_clear_event) in archive externals/libusb/libusb.a

ld: error: undefined symbol: usbi_backend
>>> referenced by core.c
>>>               core.c.o:(libusb_wrap_sys_device) in archive externals/libusb/libusb.a

ld: error: undefined symbol: usbi_backend
>>> referenced by core.c
>>>               core.c.o:(libusb_wrap_sys_device) in archive externals/libusb/libusb.a

ld: error: undefined symbol: usbi_backend
>>> referenced by core.c
>>>               core.c.o:(libusb_wrap_sys_device) in archive externals/libusb/libusb.a

ld: error: undefined symbol: usbi_backend
>>> referenced by core.c
>>>               core.c.o:(libusb_open) in archive externals/libusb/libusb.a

ld: error: undefined symbol: usbi_backend
>>> referenced by core.c
>>>               core.c.o:(libusb_open) in archive externals/libusb/libusb.a

ld: error: undefined symbol: usbi_read
>>> referenced by core.c
>>>               core.c.o:(libusb_close) in archive externals/libusb/libusb.a

ld: error: undefined symbol: usbi_write
>>> referenced by core.c
>>>               core.c.o:(libusb_close) in archive externals/libusb/libusb.a

ld: error: too many errors emitted, stopping now (use -error-limit=0 to see all errors)
c++: error: linker command failed with exit code 1 (use -v to see invocation)

2 files changed, 47 insertions, 2 deletions

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 27383bce8..cd40e5cfe 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -331,8 +331,10 @@ endif()
 
 # Ensure libusb is properly configured (based on dolphin libusb include)
 find_package(LibUSB)
-add_subdirectory(externals/libusb)
-set(LIBUSB_LIBRARIES usb)
+if (NOT LIBUSB_FOUND)
+    add_subdirectory(externals/libusb)
+    set(LIBUSB_LIBRARIES usb)
+endif()
 
 
 # Prefer the -pthread flag on Linux.
diff --git a/externals/find-modules/FindLibUSB.cmake b/externals/find-modules/FindLibUSB.cmake
new file mode 100644
index 000000000..dec0b98b0
--- /dev/null
+++ b/externals/find-modules/FindLibUSB.cmake
@@ -0,0 +1,43 @@
+# - Find libusb-1.0 library
+# This module defines
+#  LIBUSB_INCLUDE_DIR, where to find bluetooth.h
+#  LIBUSB_LIBRARIES, the libraries needed to use libusb-1.0.
+#  LIBUSB_FOUND, If false, do not try to use libusb-1.0.
+#
+# Copyright (c) 2009, Michal Cihar, <michal@cihar.com>
+#
+# vim: expandtab sw=4 ts=4 sts=4:
+
+if(ANDROID)
+       set(LIBUSB_FOUND FALSE CACHE INTERNAL "libusb-1.0 found")
+       message(STATUS "libusb-1.0 not found.")
+elseif (NOT LIBUSB_FOUND)
+    pkg_check_modules (LIBUSB_PKG libusb-1.0)
+
+    find_path(LIBUSB_INCLUDE_DIR NAMES libusb.h
+       PATHS
+       ${LIBUSB_PKG_INCLUDE_DIRS}
+       /usr/include/libusb-1.0
+       /usr/include
+       /usr/local/include/libusb-1.0
+       /usr/local/include
+    )
+
+    find_library(LIBUSB_LIBRARIES NAMES usb-1.0 usb
+       PATHS
+       ${LIBUSB_PKG_LIBRARY_DIRS}
+       /usr/lib
+       /usr/local/lib
+    )
+
+    if(LIBUSB_INCLUDE_DIR AND LIBUSB_LIBRARIES)
+       set(LIBUSB_FOUND TRUE CACHE INTERNAL "libusb-1.0 found")
+       message(STATUS "Found libusb-1.0: ${LIBUSB_INCLUDE_DIR}, ${LIBUSB_LIBRARIES}")
+    else(LIBUSB_INCLUDE_DIR AND LIBUSB_LIBRARIES)
+       set(LIBUSB_FOUND FALSE CACHE INTERNAL "libusb-1.0 found")
+       message(STATUS "libusb-1.0 not found.")
+    endif(LIBUSB_INCLUDE_DIR AND LIBUSB_LIBRARIES)
+
+    mark_as_advanced(LIBUSB_INCLUDE_DIR LIBUSB_LIBRARIES)
+endif ()
+