configure: Check for -Werror parameters on clang

Signed-off-by: Martin Storsjö <martin@martin.st>
(cherry picked from commit 9eded0fe412e610ee8944681d5c554b723463e96)
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
lavf: Fix leftovers from the ff_get_buffer patch

The automated script did not perfectly replace all the instances nor
added internal.h in all the files requiring it.