libdill/fd.c uses dill_ctx_fd without guarding it with DILL_SOCKETS

[sleeptightAnsiC]

Hi,

I attempted to compile libdill without build system
(just cc every source file and link it by hand)
and found out that compilation of libdill/fd.c fails due to:

$ cc fd.c -o fd.o -c ; cc --version ; uname -srm
fd.c: In function ‘dill_fd_allocbuf’:
fd.c:59:43: error: ‘struct dill_ctx’ has no member named ‘fd’
   59 |     struct dill_ctx_fd *ctx = &dill_getctx->fd;
      |                                           ^~
fd.c: In function ‘dill_fd_freebuf’:
fd.c:71:43: error: ‘struct dill_ctx’ has no member named ‘fd’
   71 |     struct dill_ctx_fd *ctx = &dill_getctx->fd;
      |                                           ^~
cc (GCC) 14.2.1 20240910
Linux 6.12.8-arch1-1 x86_64

It's because struct dill_ctx_fd is defined conditionally here:

libdill/ctx.h

Lines 40 to 42 in 32d0e8b

	#if defined DILL_SOCKETS
	struct dill_ctx_fd fd;
	#endif

but its usage in libdill/fd.c is NOT guarder by DILL_SOCKETS

This is of course easy to workaround with $ cc fd.c -c -DDILL_SOCKETS
but I fought it would be nice to report since it's the only issue (with #234 )
preventing libdill from being compiled without it's build system.

For comparison, there is a similar macro DILL_THREADS
which does not need to be defined in order to compile libdill.