Fix ubuntu 20 build because of underlying type mismatch

[vale1410]

Under ubuntu 20.04 and compilation as described in the readme I ran into:

In file included from /home/valentin/code/sat/qute/src/dependency_manager_rrs.cc:1:
/home/valentin/code/sat/qute/src/dependency_manager_rrs.hh:19:6: error: enumeration previously declared with fixed underlying type
enum ConstraintType;
     ^
/home/valentin/code/sat/qute/src/solver_types.hh:43:6: note: previous declaration is here
enum ConstraintType: unsigned short { clauses = 0, terms = 1 };
     ^
1 error generated.

I had to make the type of the enum explicit and the compilation worked.
https://stackoverflow.com/questions/42766839/c11-enum-forward-causes-underlying-type-mismatch#42768812

[fslivovsky]

Thanks for pointing this out! I think we don't need the forward declaration here and should instead include "solver_types.hh". Which compiler and version of CMake are you using? I couldn't reproduce the error on Ubuntu 20.04.1 LTS using GCC 9.3.0 and CMake 3.16.3.

[vale1410]

Looks like my c/c++ compiler was set to Clang-10. Still something to look into maybe!
CMakeCache.txt

[fslivovsky]

Removing the inconsistent forward declarations fixed the problem with Clang on macOS, I expect this also works for Ubuntu.