[Merge][CMake] Merged revisions : 1722

Revision  r1722
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find_package(SuperLU) failed with an error if SuperLU was not found.
This was due to the fact that find_<lib|file>(varname ...) sets
${varname} to varname-NOTFOUND if nothing is found.
Therefore using the variable varname in
CMAKE_REQUIRED_<LIBRARIES|INCLUDES> is a really bad idea.

We are preventing this now by checking the variables first.

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[[Imported from SVN: r1851]]