Use `ALGEBRA_PLUGINS_USE_SYSTEM_LIBS` across the board for configuration

[wermos]

Addresses #82.

I used ALGEBRA_PLUGINS_USE_SYSTEM_LIBS everywhere (i.e. for Vc, Fastor, and Eigen) so that now there is no surprise about why, say, Fastor is searching for a system library even though -DALGEBRA_PLUGINS_USE_SYSTEM_LIBS=TRUE was passed.