Configure carl to use our version of Eigen

[sjunges]

Currently, carl may compile with an outdated version of Eigen on the system. For Storm, we currently always ship our own. Would it make sense to use this version for carl, when we are configuring/building carl from within storm?

Relates to https://github.com/moves-rwth/carl-storm/issues/17

[sjunges]

note, that the linker in the CI also warns on a related issue:

 /opt/storm/build/include/resources/3rdparty/StormEigen/Eigen/src/Core/Stride.h:44:7: warning: type 'struct Stride' violates the C++ One Definition Rule [-Wodr]
#14 565.6    44 | class Stride
#14 565.6       |       ^
#14 565.6 /usr/include/eigen3/Eigen/src/Core/Stride.h:48:7: note: a different type is defined in another translation unit

@tquatmann