[plt-scheme] extensions on windows and missing mzdyn.exp

Hi there,

It seems that in 206p1 its necessary to explicitly export 
scheme_initialize, scheme_initialize_internal ,scheme_reload and 
scheme_module_name when building an extension in MSVC. In 205 a 
mzdyn.exp file was provided. Not sure if this is intentional.

At any rate, it would be nice to have some more details on how to link 
MzScheme extensions on Windows using MSVC. The docs only mention the 
need to link with mzdyn.o but it is also necessary to link with 
libmzsch<version>.lib and libmzgc<version>.lib as well as export the 4 
functions above (this can be done by for example adding 
/EXPORT:scheme_initialize /EXPORT:scheme_initialize_internal 
/EXPORT:scheme_reload /EXPORT:scheme_module_name to the linker 
command-line options).

btw, thanks for another great release.

-pp