vlc: fix libgcrypt support

This patch allows to override the libgcrypt-config location instead of
using the one found in the PATH, which is usually the host system one.

Fixes:
  http://autobuild.buildroot.org/results/5e4/5e42d1139bbbed2421193d8acc52df9442c43730/build-end.log



Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>