cmake: Use cmake's built-in TestForSSTREAM when testing for sstream.
cmake: Use cmake's built-in TestForSSTREAM when testing for sstream.

The original test for SSTREAM actually tested for STRSTREAM.  Instead
of fixing the logic to test for SSTREAM, it seemed easier just to use
cmake's built-in macro to perform the test.

orocos-toolchain/log4cpp,orocos-toolchain/log4cpp