gc_heap_err_str and load_image_header() are never used outside of
gc_heap.c but they are not marked static and are effectively exported
by Chibi's shared library. Since this is unlikely to be intentional,
let's hide them.
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.
