configure: Check for nanosleep in headers as well, not only in libs

On mingw64 with c++11 support, the link libraries do contain a
nanosleep function, while it isn't exposed via the headers. Using
check_func_headers instead of a plain check_func fixes this
misdetection.

Suggested-by: Hendrik Leppkes <h.leppkes@gmail.com>
See: [FFmpeg-devel] [PATCH] fix: 'make' with mingw32
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
x86/ttadsp: remove an unnecessary mova

Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
x86/dca: remove unused header

Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
vp9/x86: fix bug in intra_pred_hd_32x32.

Fixes mismatch in first keyframe in sample
ffvp9_fails_where_libvpx.succeeds.webm from ticket 3849. There's still
a second mismatch a few frames into the sample.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Remove useless configure option "--enable-sram".