g2o library doesn't export suitesparse related include headers

In a client (user) code of this library I need to depend on suitesparse explicitly, `find_package` and add its `_INCLUDE_DIRS` because g2o has headers that include suitesparse headers, but it doesn't export the suitesparse `_INCLUDE_DIRS`.



E.g.:



If I use a `LinearSolverCSparse`, I need to `#include <g2o/solvers/csparse/linear_solver_csparse.h>`.

This header includes `csparse_helper.h`:

https://github.com/RainerKuemmerle/g2o/blob/master/g2o/solvers/csparse/linear_solver_csparse.h#L30

Which includes `cs.h` from suitesparse:

https://github.com/RainerKuemmerle/g2o/blob/master/g2o/solvers/csparse/csparse_helper.h#L23



Since `LinearSolverCSparse` it's a template class, it's not trivial (if possible) to avoid this situation.

Either way, that's not wrong, the problem is that because of this, any client (user) code using this, needs to add the suitesparse headers path. The g2o library should export/add that to its `cflags` (I think), so `-I` has the headers path (as `-L` for libraries); if I knew how to do it, I'd create a PR.



@RainerKuemmerle What do you think?



FYI @v-mehta

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