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>
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