avutil: remove timer.h include from internal.h

Added libavutil/timer.h include to all files with {START,STOP}_TIMER.
mpeg12: check scantable indices in all decode_block functions

Add checks to the fast functions used with CODEC_FLAGS2_FAST and move
the check for all other functions to before the invalid memory is
accessed. Fixes https://trac.videolan.org/vlc/ticket/9713 with
CODEC_FLAGS2_FAST.

CC: libav-stable@libav.org
configure: add missing x86 dependency for i686
configure: clang: explicitly state dep file and rule name in DEPFLAGS

Fixes dependency file generation with gas-preprocessor.pl and clang.
Flags copied from GCC and tested with Apple's clang from Xcode 5 and
5.1 and clang 3.2, 3.3, 3.4 on Linux.