Correct Find Static HDF5

I got some issues on precise with cmake 2.8.5 stating there would be *only* a static hdf5 lib installed.

- `string(FIND ...)` returns the position of the sub string or `-1`
  http://www.cmake.org/cmake/help/v2.8.8/cmake.html#command:string
- the substring "hdf5.a" uses a special character `.` which has to be escaped *twice* by `\`'s
- `-1` is not one of the known boolean values (at least in cmake 2.8.5)
  http://www.cmake.org/Wiki/CMake/Language_Syntax#CMake_supports_boolean_variables.