Patch to enable compilation on hurd

[coldtobi]

For Debian I patched drizzle to compile on hurd: The issue is that PATH_MAX is not defined on this arch.

(side note: While updating the patch to the current trunk I saw that in plugin/myisam/mi_open.cc realpath (3) is called with NULL as resolved_path parmeter. This is only safe on POSIX.1-2008 or later, but there is no guarantee that this is implemented already in all your audience's enviorments. It has also been confirmed on IRC in ##posix that this is not a safe assumption and needs to be checked by feature checks.

[coldtobi (tobi-coldtobi)]

PS: I just saw that trunk has a new usage of PATH_MAX in plugin/mysql_unix_socket_protocol/protocol.cc ...
so the above patch is incomplete :-(