Fix build on recent FreeBSD (ie. upcoming FreeBSD 8.0; FreeBSD CURRENT).

Since March 9, 2009; the FreeBSD operating system base ships with libusb.  As a consequence, the devel/libusb ports that used to provide userland programs access to USB devices is useless and so marked as IGNORE for recent versions of FreeBSD.  This patch detects the FreeBSD version configure is run on and conditionally ignore the libusb detection if relevant.

Obtained from:	Ooo vcs.

configure.ac

@@ -102,11 +102,34 @@ then
 fi
 AM_CONDITIONAL(DOC_ENABLED, [test x"$enable_doc" = xyes])
 
+AC_ARG_WITH([os-version], [AS_HELP_STRING([--with-os-version], [For FreeBSD users, use this option to override the detected OSVERSION.])])
+
+case "$build_os" in
+  freebsd*)
+    AC_MSG_CHECKING([the FreeBSD operating system release])
+    if test -n "$with_os_version"; then
+      OSVERSION="$with_os_version"
+    else
+      OSVERSION=`/sbin/sysctl -n kern.osreldate`
+    fi
+    AC_MSG_RESULT([$OSVERSION])
+    AC_MSG_CHECKING([which usb library to use])
+    if test "$OSVERSION" -lt "800069"; then
+      system_has_libusb="no"
+      AC_MSG_RESULT([devel/libusb])
+    else
+      system_has_libusb="yes"
+      AC_MSG_RESULT(system)
+      LIBUSB_LIBS="-lusb"
+    fi
+esac
+
 
 # Dependencies
 PKG_CONFIG_REQUIRES=""
 ## libusb
 if test "x$enable_libusb" = "xyes"; then
+  if test x"$system_has_libusb" != "xyes"; then
   if test x"$PKG_CONFIG" = "x"; then
     AC_PATH_PROG(LIBUSB_CONFIG,libusb-config)
     if test x"$LIBUSB_CONFIG" = "x" ; then
@@ -127,6 +150,7 @@ if test "x$enable_libusb" = "xyes"; then
   if test x"$WITH_USB" = "x0"; then
     AC_MSG_ERROR([libusb is mandatory.])
   fi
+  fi
   AC_SUBST(LIBUSB_LIBS)
   AC_SUBST(LIBUSB_CFLAGS)
 fi