package/v4l2grab: update version to fix build

v4l2grab uses clock_gettime which is provided by -lrt, so we must check
and link with it.

Also, silence autoreconf when outside a git tree.

Fixes:
    http://autobuild.buildroot.org/results/9df/9df2c09963b97524621f8323b279b8a1672d4b6c/
    http://autobuild.buildroot.org/results/448/4484bda010442033d0536a95f45370701c071f92/
    http://autobuild.buildroot.org/results/b1c/b1ca0c984ec51518a4090039a86a1814582c3c2b/


    ...

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Volkov Viacheslav <sv99@inbox.ru>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>