Build position independent code when compiling Ceres statically.

- Previously, when Ceres was built as a static library we did not
  compile position independent code.  This means that the resulting
  static library could not be linked against shared libraries, but
  could be used by executables.
- To enable the use of a static Ceres library by other shared libraries
  as reported in [1], the static library must be generated from
  position independent code (except on Windows, where PIC does not
  apply).

[1] https://github.com/Itseez/opencv_contrib/pull/290#issuecomment-130389471

Change-Id: I99388f1784ece688f91b162d009578c5c97ddaf6