lib: Use gpfs.h from third_party on Linux

lib: Use gpfs.h from third_party on Linux

Update the logic for finding the gpfs.h header file: Look for the header
file in the default location under /usr/lpp/mmfs/include. If it is not
available there, default to the file in third_party/gpfs/ on Linux (AIX
could be added if there is demand).

The configure option --with-gpfs=GPFS_HEADERS_DIR can always be used to
overwrite the default behavior with a specific location.

Signed-off-by: Christof Schmitt <cs@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>
Autobuild-User(master): Volker Lendecke <vl@samba.org>
Autobuild-Date(master): Fri Jan 18 17:21:02 CET 2019 on sn-devel-144