headers: merge stralign_s.h into stralign.h

stralign_s.h is only used by stralign.h, and it's actually not part of
CRT headers. Plus, stralign_s.h is not provided by MSVC UCRT, so it can
be dropped safely.

Note that our stralign.h differs from MSVC UCRT's stralign.h a lot,
which may need further adjustments.

Also updated configure{,.ac} to remove stralign_s.h.

Signed-off-by: Zhongteng Gui <dragon-archer@outlook.com>
Signed-off-by: LIU Hao <lh_mouse@126.com>