BOOTSTRAP and libc

[Rolf Schroedter]

Could somebody please enlighten me what is the idea of libc/*.c files ?
What does it mean that if -DBOOTSTRAP uses the standard C-library,
and -UBOOTSTRAP uses the openbios-libc ?

I'm asking, because I want to embed a minimal FCode interpreter into a 
C-application,
which of course itself uses the standard C-library.

Thanks & regards,
Rolf.