package/ndisc6: needs c11

package/ndisc6: needs c11

ndisc needs c11 because of alignas since version 1.0.4 and
https://git.remlab.net/gitweb/?p=ndisc6.git;a=commit;h=d917e5172531dd72e896b317c07f64274e8024a1
https://git.remlab.net/gitweb/?p=ndisc6.git;a=commit;h=77a2efc350fdd52b76b1a4c874ec4029e722048b

So replace -std=gnu99 by -std=c11 and add a dependency on gcc 4.7

This will fix build with gcc 8

Fixes:
 - http://autobuild.buildroot.org/results/1cab1e5f7c4250a153fd1ab758f818078833972f

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>