Add checks for more Eigen headers at configure

[Prof Garth Wells]

A user who just copies the Eigen header files into, for example, /usr/local/include, won't have eigen3/unsupported.

The build system should check for the directory ../unsupported.

[Prof Garth Wells]

Check for 'unsupported' Eigen headers during configuration. Fixes Issue #541.

We use some 'unsupported' header files to access some Krylovsolver.

→ <<cset 593a7d47ba5c>>

[Martin Sandve Alnæs]

Removing milestone: 1.6 (automated comment)