*_API to specify linkage for lib/cgraph on Windows

*_API to specify linkage for lib/cgraph on Windows

Instead of overloading the `extern` keyword with __dllspec(dllexport) or
__dllspec(dllimport) to specify the linkage, and use a function definition
file to specify the exported functions, this commit adds a seperate define
for each header file to do both. It also makes it more explicit which
function are exported.