Added CMake probe for GSL if requested via the DAKOTA_HAVE_GSL varialble. If GSL support is requested, and NOT installed in a system area, then an environment variable is used to locate GSL, i.e.
Added CMake probe for GSL if requested via the DAKOTA_HAVE_GSL varialble.
If GSL support is requested, and NOT installed in a system area, then
an environment variable is used to locate GSL, i.e.

export GSL_CONFIG=/path/to/gsl_prefix/bin/gsl-config
then invoke cmake normally, with -DDAKOTA_HAVE_GSL=ON argument

ToDo:  (Short-term) more testing/improve/cleanup
       (Long-term) Windows implementation of GSL probe
                     (likely leverage CMake "SuperBuild")

For traceability, partial "fix" for Trac ticket #972.
Request review by Brian.



git-svn-id: dcef21d3ff4cbd8dced82702dcaad637bf246ecc@973 aa0779c3-6ec2-4b84-a212-a405c134cd8d

pemryan/DAKOTA,pemryan/DAKOTA,pemryan/DAKOTA,pemryan/DAKOTA,pemryan/DAKOTA,pemryan/DAKOTA,pemryan/DAKOTA