implicit function declaration warnings

Hello,
While packaging Scheme 48 for Gentoo, I found several implicit function
declaration warnings.  I posted a patch to fix three of them.  There are
two more that I haven't fixed:

c/net/socket.c:116:3: warning: implicit declaration of function ``s48_os_error'' [-Wimplicit-function-declaration]
c/unix/socket.c:44:3: warning: implicit declaration of function ``s48_os_error'' [-Wimplicit-function-declaration]

The declaration of s48_os_error isn't available in these two files,
because NO_OLD_FFI is defined when scheme48.h is included.

-- Chris