Allow disabling libsysfs support

Add a configure switch to allow disabling libsysfs support instead of letting it check what is on the build host and enable it accordingly. The default behavior is unchanged.
Signed-off-by: Christophe Mutricy <xtophe@videolan.org>
(cherry picked from commit 31d3d277)

configure.ac

@@ -3281,9 +3281,14 @@ AC_CHECK_HEADERS(zlib.h, [
   VLC_ADD_LIBS([mp4 skins2 sap mkv gme],[-lz])
 ] )
 
-AC_CHECK_HEADERS(sysfs/libsysfs.h, [
-  VLC_ADD_LIBS([mp4 mkv],[-lsysfs])
-] )
+AC_ARG_ENABLE(libsysfs,
+  [  --enable-libsysfs       Get user key from firewire connected iPod via libsysfs (default enabled)])
+
+AS_IF([test "${enable_libsysfs}" != "no"],[
+  AC_CHECK_HEADERS(sysfs/libsysfs.h, [
+    VLC_ADD_LIBS([mp4 mkv],[-lsysfs])
+  ] )
+])
 
 dnl
 dnl skins2 module