Tweak the libusb detection.

1. Try to use pkg-config to locate libusb;
  2. Try with libusb-config if not found;
  3. Try to locate headers and lib if still not found.

This would hopefully prevent build failures in some very particular cases, such as cross compiling libnfc on a FreeBSD version for another FreeBSD version that does not use the same libusb.

http://www.freebsd.org/cgi/query-pr.cgi?pr=ports/142836

Reported by:	Dmitry Marakasov

configure.ac

@@ -109,44 +109,14 @@ 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"$LIBUSB_CONFIG" = "x" ; then
   WITH_USB=0
-    else
-      LIBUSB_CFLAGS=`$LIBUSB_CONFIG --cflags`
-      LIBUSB_LIBS=`$LIBUSB_CONFIG --libs`
-      WITH_USB=1
-    fi
-  else
+
+  # Search using pkg-config
+  if test x"$PKG_CONFIG" != "x"; then
     PKG_CHECK_MODULES(LIBUSB, libusb, [WITH_USB=1], [WITH_USB=0])
     if test x"$PKG_CONFIG_REQUIRES" != x""; then
       PKG_CONFIG_REQUIRES="$PKG_CONFIG_REQUIRES,"
@@ -154,10 +124,29 @@ if test "x$enable_libusb" = "xyes"; then
     PKG_CONFIG_REQUIRES="$PKG_CONFIG_REQUIRES libusb"
   fi
 
+  # Search using libusb-config
+  if test x"$WITH_USB" = "x0"; then
+    AC_PATH_PROG(LIBUSB_CONFIG,libusb-config)
+    if test x"$LIBUSB_CONFIG" != "x" ; then
+      LIBUSB_CFLAGS=`$LIBUSB_CONFIG --cflags`
+      LIBUSB_LIBS=`$LIBUSB_CONFIG --libs`
+      WITH_USB=1
+    fi
+  fi
+
+  # Search the library and headers directly (last chance)
+  if test x"$WITH_USB" = "x0"; then
+    AC_CHECK_HEADER(usb.h, [], [AC_MSG_ERROR([The libusb headers are missing])])
+    AC_CHECK_LIB(usb, libusb_init, [], [AC_MSG_ERROR([The libusb library is missing])])
+
+    LIBUSB_LIBS="-lusb"
+    WITH_USB=1
+  fi
+
   if test x"$WITH_USB" = "x0"; then
     AC_MSG_ERROR([libusb is mandatory.])
   fi
-  fi
+
   AC_SUBST(LIBUSB_LIBS)
   AC_SUBST(LIBUSB_CFLAGS)
 fi