Make use of seteuid() determined by configure.

clientserver.c

@@ -809,7 +809,11 @@ static int rsync_module(int f_in, int f_out, int i, const char *addr, const char
 	}
 
 	if (set_uid) {
-		if (setuid(uid) || seteuid(uid)) {
+		if (setuid(uid) < 0
+#ifdef HAVE_SETEUID
+		 || seteuid(uid) < 0
+#endif
+		) {
 			rsyserr(FLOG, errno, "setuid %ld failed", (long)uid);
 			io_printf(f_out, "@ERROR: setuid failed\n");
 			return -1;

configure.in

@@ -572,7 +572,7 @@ AC_CHECK_FUNCS(waitpid wait4 getcwd strdup chown chmod lchmod mknod mkfifo \
     memmove lchown vsnprintf snprintf vasprintf asprintf setsid strpbrk \
     strlcat strlcpy strtol mallinfo getgroups setgroups geteuid getegid \
     setlocale setmode open64 lseek64 mkstemp64 mtrace va_copy __va_copy \
-    strerror putenv iconv_open locale_charset nl_langinfo getxattr \
+    seteuid strerror putenv iconv_open locale_charset nl_langinfo getxattr \
     extattr_get_link sigaction sigprocmask setattrlist getgrouplist \
     initgroups utimensat)