add example how-to for generating .o files and using them to compile

[lowagner]

i'm not sure this is cross platform, but adding a "-c" option will generate ".o" files for gcc.

also i'm not a C expert so there may be holes in how dependency stuff works, so please add comments if i'm missing something.

[fbucafusco]

for cross compile, check this out #127

[rexim]

This example does not demonstrate any new aspects of nob.h that are not covered by existing example. -c flag is the aspect of a C compiler, not nob.h.

[lowagner]

Is there not a cross-platform way to do -c? It would be nice to have such an example.