Escape backquote in help text

Otherwise this will of course be interpreted by the shell, which is not
the intention here.
Let SuiteSparse path be specified without config mode

Use the _ROOT suffix to direct the CMake module to use a particular path
to the SuiteSparse installation without also triggering config mode (by
convention CMake uses Foo_DIR as the name of the variable which
specifies where to look for another *project*).
Allow umfpack to be used as an alias for SuiteSparse

Most people will associate the use of SuiteSparse with "using UMFPACK"
since that is the component that is actively used.
Allow system installation to be used with dunecontrol
Annotate dependencies with version number