Add standard variables/location hints

There is a convention that one can do `pkg-config --variable=includedir`
and have it return the path to the source files (even if that should
have been taken care of by options in --cflags).
Generate libtool archive (.la) files

These files are text files which specify which platform specific
libraries we should be looking for. We need libtool installed to
use them, but they can be generated pretty inexpensively from the
CMake script itself.
Write information file for libtool
Link to core function to get more realistic test

Using umfpack_dl_solve as a test function increases the chance that the
probe will not be able to link successfully if it has not found all
libraries necessary (to link other than trivial test programs).
Use same default as DUNE; build as a static library

Packages should build with -DBUILD_SHARED_LIBS=ON