[buildsystem] Make dune-cornerpoint a suggestion.
do not add the "-Wl,--as-needed" linker flag in FindSuiteSparse.cmake

because if umfpack is fully linked but some other library is
underlinked, it causes the build to fail. This flag is better dealt
with in the "UseOnlyNeeded.cmake" module...
make the "-Wl,--as-needed" flag optional

as there is not much harm done if it is not used but it may cause the
build to fail if it is present (read: it causes _my_ build to fail),
it defaults to OFF.
Fix option name to match name in use in conditional.

Also add a useful doc string.
Merge pull request #580 from atgeirr/fix-external-warnings-option

Fix option name to match name in use in conditional.
Merge pull request #577 from andlaus/make_as_needed_optional

Make as needed optional
Merge pull request #576 from bska/sync-build

[buildsystem] Make dune-cornerpoint a suggestion.