Avoid implicit declaration of isfdtype, for C99 compatibility.

Partially fixed upstream via:

commit 7ebdf2cd6232aa22c78fd9b59951a3e9ee43a715
Author: Ralph Little <skelband@gmail.com>
Date:   Tue Apr 6 09:27:09 2021 -0700

    scanadf: fixes for various compilation warnings

Rest submitted upstream:

  <https://gitlab.com/sane-project/frontends/-/merge_requests/9>

diff --git a/configure b/configure
index 663bf8b..804634b 100755
--- a/configure
+++ b/configure
@@ -4507,7 +4507,7 @@ fi
 
 
 
-for ac_header in os2.h fcntl.h unistd.h libc.h sys/time.h sys/types.h
+for ac_header in os2.h fcntl.h unistd.h libc.h sys/time.h sys/types.h sys/socket.h
 do
 as_ac_Header=`echo "ac_cv_header_$ac_header" | $as_tr_sh`
 if eval "test \"\${$as_ac_Header+set}\" = set"; then
diff --git a/configure.in b/configure.in
index ef43528..06dd75d 100644
--- a/configure.in
+++ b/configure.in
@@ -75,7 +75,8 @@ esac
 
 dnl Checks for header files.
 AC_HEADER_STDC
-AC_CHECK_HEADERS(os2.h fcntl.h unistd.h libc.h sys/time.h sys/types.h)
+AC_CHECK_HEADERS(os2.h fcntl.h unistd.h libc.h sys/time.h sys/types.h
+		 sys/socket.h)
 
 dnl Checks for typedefs, structures, and compiler characteristics.
 AC_C_CONST
diff --git a/include/sane/config.h.in b/include/sane/config.h.in
index baf35be..b8d901d 100644
--- a/include/sane/config.h.in
+++ b/include/sane/config.h.in
@@ -108,6 +108,9 @@
 /* Define to 1 if you have the `strtod' function. */
 #undef HAVE_STRTOD
 
+/* Define to 1 if you have the <sys/socket.h> header file. */
+#undef HAVE_SYS_SOCKET_H
+
 /* Define to 1 if you have the <sys/stat.h> header file. */
 #undef HAVE_SYS_STAT_H
 
-- 
2.39.0