LUDOC-211 Makefile: Added Makefile support for SLED 11 SP2

LUDOC-211 Makefile: Added Makefile support for SLED 11 SP2

Added a variable XSL_SLE to the makefile which points at the location
of the docbook.xsl files in SLE distributions.  Added the check on
whether that directory exists in the XSL variable assignment.

Tested that "make html" works on a SLED 11 SP2 machine.

Signed-off-by: Ryan Haasken <haasken@cray.com>
Change-Id: I82f0407ea98904c2060090d3d205a36f16d2b153
Reviewed-on: http://review.whamcloud.com/8493
Tested-by: Jenkins
Reviewed-by: Ann Koehler <amk@cray.com>
Reviewed-by: Patrick Farrell <paf@cray.com>
Reviewed-by: Chris Horn <hornc@cray.com>
Reviewed-by: Richard Henwood <richard.henwood@intel.com>