mariadb-10.1 FTBFS on GNU/Hurd due to use of PATH_MAX

Attached are patches to make mariadb-10.1 to build on GNU/Hurd. Unfortunately
there are still some warnings to fix and the testsuite fails, and has been
removed with debian/rules patch. Attached is also an updated symbols file, the
link given in debian/ points to a file having three symbols not in the Hurd
version.
The patched package has also been successfully built on GNU/Linux amd64 with
pbuilder.cmake.patch defines the system name GNU, tries to disable dtrace (which is Linux
only from systemtap-sdt-dev) and add a check for HAVE_SYS_POLL_H, see
path_max.patch. Unfortunately disabling dtrace did not work, I had to uninstall
systemtap-sdt-dev, I wonder why?
path_max.patch defines GNU_SOURCE if not already defined, avoids a FreeBSD-
specific definition of O_PATH, and fixes PATH_MAX issues in files
/mysys/mysys_priv.h, sql/wsrep_binlog.cc and extra/mariabackup/backup_copy.cc.
The POLL stuff in storage/mroonga/vendor/groonga/lib/grn_com.h and
storage/mroonga/vendor/groonga/lib/com.c fixes the USE_POLL case. That case is
not properly handled in the upstream code. GNU/Hurd could of course use
USE_SELECT, but since the OS is detected as using POLL, that choice is fixed
too.

The current debian version is at 10.1.23-6. Since there is a freeze going on and since the GNU patches are also affecting other OSes it would be nice if the patches are accepted upstream. If so, they are easily motivation to be accepted in Debian, including the imminent Stretch release.