Correct Find Static HDF5

- string(FIND ...) returns the position of the sub string or -1
  http://www.cmake.org/cmake/help/v2.8.8/cmake.html#command:string
- -1 is not one of the known boolean values (at least in 2.8.5)
  http://www.cmake.org/Wiki/CMake/Language_Syntax#CMake_supports_boolean_variables.
Merge pull request #71 from ax3l/fix/findStatic

Correct Find Static HDF5