FindSoci.cmake: fix couple of minor issues

- there is no SOCI_${plugin}_LIBRARY variable, correct name is
  SOCI_${plugin}_PLUGIN
- variable names, not variable expansions, shold be placed into
  confitional statements like IF()
Merge pull request #40 from mloskot/master

Bumped version number to 3.2.0
Merge pull request #36 from avg-I/cmake/find-soci

FindSoci.cmake: fix couple of minor issues
Merge branch 'master' of https://github.com/SOCI/soci