Try to fix win32 port. Now it runs both on Win32/Win64.
Win64 port currently depends on 128bits arithmetic thus it does not run on
MSVC.
Makefile now have EXCLUDE_POSIX_LIBS knob to exclude posix related library
from build.
Introduce msys PLATFORM for Makefile.detect to use MSYS's POSIX
emulation layer. It is intended for linking against MSYS tools; it is
not for embedding to Win32 applications.
First we check for C99 support in Makefile.detect, looking for the
header we need and verifying whether it is the right one by using
a definition required by C99 standard to be present in that header.
uintN_t types are optional, but implementations are required to
provide corresponding limit #defines for the types they support,
so we can check for this with preprocessor only.
Finally, we define SEXP_UINTN_DEFINED for any sexp_uintN_t we have
so that the code can use #ifs to check for exact integer support.
