Uniform external library support

Submitted by Benoit Jacob

Assigned to Gael Guennebaud

Link to original bugzilla bug (#57)

Description

[Gael: i'm filing this under Sparse as I'm not sure if you mean this only for sparse solver back-ends.]

Via a preprocessor token? a dedicated (sub)module? automatically enable it if a header of the respective library has been included?

Blocking

#55 (closed)