Add a declaration check for CMSG_DATA to mod_proxy_fdpass.

Add a declaration check for CMSG_DATA to mod_proxy_fdpass.
Also move '#error' after the includes, because it seems
CMSG_DATA is commonly defined in sys/socket.h.

The check also overwrites _apmod_error_fatal in case
CMSG_DATA is not defined to allow building when configure
was called with "--enable-proxy". Otherwise one would need
to explicitly disable mod_proxy_fdpass.

We might want to remove the #error completely, because the
new feature test prevents the building of the module when
the symbol is not defined.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@730803 13f79535-47bb-0310-9956-ffa450edef68