common/filestuff.c: No sockets on DJGPP.

Building gdb with --host=i586-pc-msdosdjgpp ends up with:

 i586-pc-msdosdjgpp-gcc -g -O2 -I../../src/gdb/config/djgpp  -I. -I../../src/gdb -I../../src/gdb/common -I../../src/gdb/config -DLOCALEDIR="\"/usr/local/share/locale\"" -DHAVE_CONFIG_H -I../../src/gdb/../include/opcode -I../../src/gdb/../opcodes/.. -I../../src/gdb/../readline/.. -I../bfd -I../../src/gdb/../bfd -I../../src/gdb/../include -I../libdecnumber -I../../src/gdb/../libdecnumber -I./../intl -I../../src/gdb/gnulib/import -Ibuild-gnulib/import    -Wall -Wdeclaration-after-statement -Wpointer-arith -Wformat-nonliteral -Wpointer-sign -Wno-unused -Wunused-value -Wunused-function -Wno-switch -Wno-char-subscripts -Wmissing-prototypes -Wdeclaration-after-statement -Wempty-body -Werror -c -o filestuff.o -MT filestuff.o -MMD -MP -MF .deps/filestuff.Tpo ../../src/gdb/common/filestuff.c
 ../../src/gdb/common/filestuff.c:38:24: fatal error: sys/socket.h: No such file or directory

There are no sockets on djgpp.  This #ifdef's out the bits in the file
that use sockets, depending on whether winsock or sys/socket.h is
available.

As alternative approach, given ser-tcp.c, ser-pipe.c, etc. are split
into separate files, and which to use is selected by configure.ac:

 dnl Figure out which of the many generic ser-*.c files the _host_ supports.
 SER_HARDWIRE="ser-base.o ser-unix.o ser-pipe.o ser-tcp.o"
 case ${host} in
  *go32* ) SER_HARDWIRE=ser-go32.o ;;
  *djgpp* ) SER_HARDWIRE=ser-go32.o ;;
  *mingw32*) SER_HARDWIRE="ser-base.o ser-tcp.o ser-mingw.o" ;;
 esac
 AC_SUBST(SER_HARDWIRE)

... I considered splitting filestuff.c similarly.  But I quickly gave
up on the idea, as it looked like a lot more complication over this
approach, for no real gain.  Plus, there are uses of these functions
outside the ser*.c framework.

gdbserver's configure.ac is already checking for sys/socket.h.

gdb/
2013-05-23  Pedro Alves  <palves@redhat.com>

	* common/filestuff.c [USE_WIN32API]: Define HAVE_SOCKETS.
	[HAVE_SYS_SOCKET_H]: Define HAVE_SOCKETS.
	(socket_mark_cloexec, gdb_socketpair_cloexec, gdb_socket_cloexec):
	Only define if HAVE_SOCKETS is defined.
	* configure.ac: Check for sys/socket.h.
	* config.in, configure: Regenerate.

gdb/common/filestuff.c

@@ -34,10 +34,12 @@
 #ifdef USE_WIN32API
 #include <winsock2.h>
 #include <windows.h>
-#else
+#define HAVE_SOCKETS 1
+#elif defined HAVE_SYS_SOCKET_H
 #include <sys/socket.h>
 /* Define HAVE_F_GETFD if we plan to use F_GETFD.  */
 #define HAVE_F_GETFD F_GETFD
+#define HAVE_SOCKETS 1
 #endif
 
 #ifdef HAVE_SYS_RESOURCE_H
@@ -277,6 +279,8 @@ maybe_mark_cloexec (int fd)
     mark_cloexec (fd);
 }
 
+#ifdef HAVE_SOCKETS
+
 /* Like maybe_mark_cloexec, but for callers that use SOCK_CLOEXEC.  */
 
 static void
@@ -286,6 +290,8 @@ socket_mark_cloexec (int fd)
     mark_cloexec (fd);
 }
 
+#endif
+
 
 
 /* See filestuff.h.  */
@@ -335,6 +341,7 @@ gdb_fopen_cloexec (const char *filename, const char *opentype)
   return result;
 }
 
+#ifdef HAVE_SOCKETS
 /* See filestuff.h.  */
 
 int
@@ -367,6 +374,7 @@ gdb_socket_cloexec (int namespace, int style, int protocol)
 
   return result;
 }
+#endif
 
 /* See filestuff.h.  */