[autobuild] allow sendfile() in cross-compile (fixes #2836)

allow sendfile() in cross-compile if sys/sendfile.h header detected
and sendfile() symbol detected (e.g. in libc)

If sendfile() is present but always returns ENOSYS, lighttpd will
be slightly less efficient, but will fall back to writev() or write()

User should explicitly set server.network-backend = "writev" in
lighttpd.conf on systems with broken sendfile()

x-ref:
  "Remove check for broken sendfile implementations"
  https://redmine.lighttpd.net/issues/2836

configure.ac

@@ -1141,9 +1141,6 @@ if test "$ac_cv_func_sendfile" = yes; then
         AC_DEFINE([HAVE_SENDFILE_BROKEN], [1], [broken sendfile])
       ]
     )
-  else
-    AC_MSG_RESULT([no, cross-compiling])
-    AC_DEFINE([HAVE_SENDFILE_BROKEN], [1], [broken sendfile])
   fi
 fi