Features from optional dependencies

sprs_suitesparse_camd implicit feature

Enables sprs_suitesparse_camd

sprs_suitesparse_camd:

sprs bindings to the suitesparse camd fill-in reducting ordering

sprs_suitesparse_ldl implicit feature

Enables sprs_suitesparse_ldl

sprs_suitesparse_ldl:

sprs bindings to the suitesparse ldl solver