make sure cap_v4l is used by default on Linux

cap_v4l is better maintained and generally the code is in
better shape than cap_libv4l.

CMakeLists.txt

@@ -194,7 +194,7 @@ OCV_OPTION(WITH_PTHREADS_PF    "Use pthreads-based parallel_for"             ON
 OCV_OPTION(WITH_TIFF           "Include TIFF support"                        ON   IF (NOT IOS) )
 OCV_OPTION(WITH_UNICAP         "Include Unicap support (GPL)"                OFF  IF (UNIX AND NOT APPLE AND NOT ANDROID) )
 OCV_OPTION(WITH_V4L            "Include Video 4 Linux support"               ON   IF (UNIX AND NOT ANDROID) )
-OCV_OPTION(WITH_LIBV4L         "Use libv4l for Video 4 Linux support"        ON   IF (UNIX AND NOT ANDROID) )
+OCV_OPTION(WITH_LIBV4L         "Use libv4l for Video 4 Linux support"        OFF  IF (UNIX AND NOT ANDROID) )
 OCV_OPTION(WITH_DSHOW          "Build VideoIO with DirectShow support"       ON   IF (WIN32 AND NOT ARM AND NOT WINRT) )
 OCV_OPTION(WITH_MSMF           "Build VideoIO with Media Foundation support" OFF  IF WIN32 )
 OCV_OPTION(WITH_XIMEA          "Include XIMEA cameras support"               OFF  IF (NOT ANDROID AND NOT WINRT) )