cabac: silence "maybe-uninitialized" compiler warning

This uses a #pragma to disable the maybe-unitialized check for
this one variable. The pragma is available from gcc 4.6. A warning
about ignoring the pragma is newly introduced in gcc 3.3.6.

This silences:
encoder/cabac.c:1048:17: note: ‘*((void *)&nnzbak+12).i’ was declared here
encoder/cabac.c:1091:17: warning: ‘*((void *)&nnzbak+4).i’ may be used uninitialized in this function [-Wmaybe-uninitialized]
encoder/cabac.c:1048:17: note: ‘*((void *)&nnzbak+4).i’ was declared here
encoder/cabac.c:1091:17: warning: ‘nnzbak[2][1]’ may be used uninitialized in this function [-Wmaybe-uninitialized]
encoder/cabac.c:1048:17: note: ‘nnzbak[2][1]’ was declared here
encoder/cabac.c:1091:17: warning: ‘nnzbak[2][0]’ may be used uninitialized in this function [-Wmaybe-uninitialized]
encoder/cabac.c:1048:17: note: ‘nnzbak[2][0]’ was declared here
encoder/cabac.c:1091:17: warning: ‘nnzbak[1][1]’ may be used uninitialized in this function [-Wmaybe-uninitialized]
encoder/cabac.c:1048:17: note: ‘nnzbak[1][1]’ was declared here
encoder/cabac.c:1091:17: warning: ‘nnzbak[1][0]’ may be used uninitialized in this function [-Wmaybe-uninitialized]
encoder/cabac.c:1048:17: note: ‘nnzbak[1][0]’ was declared here
encoder/cabac.c:1091:17: warning: ‘nnzbak[0][1]’ may be used uninitialized in this function [-Wmaybe-uninitialized]
encoder/cabac.c:1048:17: note: ‘nnzbak[0][1]’ was declared here
encoder/cabac.c:1091:17: warning: ‘nnzbak[0][0]’ may be used uninitialized in this function [-Wmaybe-uninitialized]
encoder/cabac.c:1048:17: note: ‘nnzbak[0][0]’ was declared here
encoder/cabac.c:1091:17: warning: ‘nnzbak[2][3]’ may be used uninitialized in this function [-Wmaybe-uninitialized]
encoder/cabac.c:1048:17: note: ‘nnzbak[2][3]’ was declared here
encoder/cabac.c:1091:17: warning: ‘nnzbak[2][2]’ may be used uninitialized in this function [-Wmaybe-uninitialized]
encoder/cabac.c:1048:17: note: ‘nnzbak[2][2]’ was declared here
encoder/cabac.c:1091:17: warning: ‘nnzbak[1][3]’ may be used uninitialized in this function [-Wmaybe-uninitialized]
encoder/cabac.c:1048:17: note: ‘nnzbak[1][3]’ was declared here
encoder/cabac.c:1091:17: warning: ‘nnzbak[1][2]’ may be used uninitialized in this function [-Wmaybe-uninitialized]
encoder/cabac.c:1048:17: note: ‘nnzbak[1][2]’ was declared here
encoder/cabac.c:1091:17: warning: ‘nnzbak[0][3]’ may be used uninitialized in this function [-Wmaybe-uninitialized]
encoder/cabac.c:1048:17: note: ‘nnzbak[0][3]’ was declared here
encoder/cabac.c:1091:17: warning: ‘nnzbak[0][2]’ may be used uninitialized in this function [-Wmaybe-uninitialized]
encoder/cabac.c:1048:17: note: ‘nnzbak[0][2]’ was declared here
common/bitstream.h: read initial bitstream values only if needed

This silences several compiler warnings in encoder/set.c:
encoder/set.c: In function ‘x264_sei_recovery_point_write’:
./common/bitstream.h:101:31: warning: ‘tmp_buf.i’ is used uninitialized in this function [-Wuninitialized]
encoder/set.c:544:13: note: ‘tmp_buf’ was declared here
In file included from ./common/common.h:242:0,
                 from encoder/set.c:27:
encoder/set.c: In function ‘x264_sei_buffering_period_write’:
./common/bitstream.h:101:31: warning: ‘tmp_buf.i’ is used uninitialized in this function [-Wuninitialized]
encoder/set.c:596:13: note: ‘tmp_buf’ was declared here
In file included from ./common/common.h:242:0,
                 from encoder/set.c:27:
encoder/set.c: In function ‘x264_sei_pic_timing_write’:
./common/bitstream.h:101:31: warning: ‘tmp_buf.i’ is used uninitialized in this function [-Wuninitialized]
encoder/set.c:618:13: note: ‘tmp_buf’ was declared here
In file included from ./common/common.h:242:0,
                 from encoder/set.c:27:
encoder/set.c: In function ‘x264_sei_frame_packing_write’:
./common/bitstream.h:101:31: warning: ‘tmp_buf.i’ is used uninitialized in this function [-Wuninitialized]
encoder/set.c:649:13: note: ‘tmp_buf’ was declared here
In file included from ./common/common.h:242:0,
                 from encoder/set.c:27:
encoder/set.c: In function ‘x264_sei_dec_ref_pic_marking_write’:
./common/bitstream.h:101:31: warning: ‘tmp_buf.i’ is used uninitialized in this function [-Wuninitialized]
encoder/set.c:700:13: note: ‘tmp_buf’ was declared here
common/threadpool.c: silence compiler warning

An extra cast fixes the size difference on 64bit platforms.

common/threadpool.c: In function ‘x264_threadpool_thread’:
common/threadpool.c:69:20: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
Revert "common/threadpool.c: silence compiler warning"

This reverts commit b328a555252611e9e7f3e7c7b6c978e4bb0fecb2.
Patch was too ugly.