dnl do not use LDFLAGS, since PKGCONFIG will provide everything

  saved_LDFLAGS=${LDFLAGS}

  LDFLAGS=

  LIBUSB_PKG_CONFIG_NAME=

  dnl loop over various possible 'libusb' PKGCONFIG names, and choose

  dnl the first one that meets both the user's selection (via

  dnl configure flags) as well as what is installed

  dnl create the list of libusb PKGCONFIG modules to test

  libusb_list=''

    dnl libusb-1.0 was requested; just test for it

    libusb_list="libusb-1.0"

    dnl test for legacy libusb only

    libusb_list="libusb libusb-legacy"

  fi

  for libusb_name in ${libusb_list}; do

    dnl clear internal variables

    libusbok=no

    have_libusb1=no

    usb_header=''

    usb_lib_func=''

    usb_lib_name=''

    dnl start checks

    AC_MSG_NOTICE([Checking for LIBUSB version '${libusb_name}'])

    if test ${libusb_name} = "libusb-1.0"; then

      dnl see if the pkgconfig module is available

      PKG_CHECK_MODULES(USB, ${libusb_name}, [

        libusbok=yes

        have_libusb1=yes

        usb_header='libusb-1.0/libusb.h'

        usb_lib_func='libusb_bulk_transfer'

    else

      dnl see if the pkgconfig module is available

      PKG_CHECK_MODULES(USB, ${libusb_name}, [

        usb_header='usb.h'

        usb_lib_func='usb_bulk_write'

      ], [libusbok=no])

    if test $libusbok = yes; then

      dnl PKGCONFIG found a version of LIBUSB.

      dnl Check it to make sure it meets enough criteria:

      dnl Verify that $usb_header is a valid header. If so, then

      dnl verify that $usb_lib_func can be found in the library

      dnl $usb_lib_name.  if so, verify that the symbol 'usb_debug' is

      dnl found in the library.

      dnl Check for the header.  Similar to AC_CHECK_HEADERS,

      dnl but doesn't append to known \#defines.

      dnl If PKGCONFIG found variable USB_INCLUDEDIR, and it is

      dnl not empty, use it for checking for $usb_header.

      dnl Otherwise, maybe the user's shell environment is already

      dnl configured to find this header.

      AC_LANG_PUSH(C)

      save_CPPFLAGS="$CPPFLAGS"

      if test x$USB_INCLUDEDIR != x; then

        USB_INCLUDES="-I$USB_INCLUDEDIR"

        CPPFLAGS="$USB_INCLUDES"

      fi

      AC_MSG_CHECKING([$libusb_name for header $usb_header])

      AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[

        #include "$usb_header"

        ]], [])],

        [libusbok=yes],[libusbok=no])

      AC_MSG_RESULT([$libusbok])

      CPPFLAGS="$save_CPPFLAGS"

      AC_LANG_POP(C)

      if test $libusbok = yes; then

        dnl found the header; now make sure the library is OK

        dnl On Darwin, need to include the IOKit library.     

        AC_LANG_PUSH(C)

        save_LIBS="$LIBS"

        LIBS=""

        case "$host_os" in

          darwin*)

            USB_LIBS="$USB_LIBS -lIOKit"

            LIBS="$USB_LIBS"

            ;;

          *) ;;

        esac

 	dnl find the library link name

        usb_lib_name=`echo $USB_LIBS | sed -e "s@.*-l\(usb[[^ ]]*\).*@\1@"`

        dnl Check for the function in the library.  Similar to

        dnl AC_CHECK_LIB, but doesn't append to known \#defines.

        AC_MSG_CHECKING([$libusb_name for function $usb_lib_func in library $usb_lib_name])

        AC_LINK_IFELSE([AC_LANG_PROGRAM([[

          #ifdef __cplusplus

          extern "C"

          #endif

          char $usb_lib_func ();

          ]], [[return $usb_lib_func ();]])],

          [libusbok=yes],[libusbok=no])

        AC_MSG_RESULT([$libusbok])

        case "$host_os" in

          cygwin* | mingw*)

            USB_LIBS="$LIBS"

            ;;

          *) ;;

        esac

        LIBS="$save_LIBS"

        AC_LANG_POP(C)

        if test $libusbok = yes; then

          if test ${libusb_name} != "libusb-1.0"; then

            dnl PKGCONFIG found a legacy version of libusb; make sure the

            dnl variable _usb_debug is available in the found library

            AC_LANG_PUSH(C)

            save_CPPFLAGS="$CPPFLAGS"

            CPPFLAGS="$USB_INCLUDES"

            save_LIBS="$LIBS"

            LIBS="$USB_LIBS"

            AC_MSG_CHECKING([$libusb_name for symbol usb_debug in library $usb_lib_name])

            AC_LINK_IFELSE([AC_LANG_PROGRAM([[

              extern int usb_debug;]],

              [[usb_debug = 0;]])],

              [libusbok=yes],[libusbok=no])

            AC_MSG_RESULT([$libusbok])

            LIBS="$save_LIBS"

            CPPFLAGS="$save_CPPFLAGS"

            AC_LANG_POP(C)

          fi

        fi

      fi

    fi

    dnl if everything checks out OK, finish up

    if test $libusbok = yes; then

      LIBUSB_PKG_CONFIG_NAME="${libusb_name}"

      break

    else

      dnl something wasn't found in this LIBUSB version.

      dnl HACK: clear cache variables for header and library

      unset USB_INCLUDEDIR

      unset USB_INCLUDES

      unset USB_LIBS

      unset $as_ac_Header

      unset $as_ac_Lib

    fi

  done

  if test $libusbok = yes; then

    dnl final error checking, mostly to create #define's

    CPPFLAGS="$USB_INCLUDES"

    dnl Check for the header.

      dnl check for the library (again)

      save_CPPFLAGS="$CPPFLAGS"

      CPPFLAGS="$USB_INCLUDES"

      LIBS="$USB_LIBS"

        AC_MSG_RESULT([USRP requires library '$usb_lib_name' with function '$usb_lib_func', which was either not found or was not usable. See http://www.libusb.org])])

      CPPFLAGS="$save_CPPFLAGS"

    dnl success

    AC_MSG_NOTICE([Using LIBUSB version '${libusb_name}'])

    dnl not found; clear substitution variables

    LIBUSB_PKG_CONFIG_NAME=

  dnl create substitution variables

  AC_SUBST(USB_INCLUDES)

  AC_SUBST(USB_LIBS)

  AC_SUBST(LIBUSB_PKG_CONFIG_NAME)

  dnl restore LDFLAGS

  LDFLAGS=${saved_LDFLAGS}