To compile libfactorial.dylib on OS X:
$ gcc -dynamiclib -undefined suppress -flat_namespace factorial.c -o libfactorial.dylib
To compile libfactorial.so on Linux/Solaris/etc.:
$ gcc -shared -fpic factorial.c -o libfactorial.so
To compile libfactorial.dll on Windows (http://stackoverflow.com/a/2220213):
$ cl.exe /D_USRDLL /D_WINDLL factorial.c /link /DLL /OUT:libfactorial.dll
To run the example:
$ node factorial.js 35
Your output: 6399018521010896896