Put HAVE_DYNAMIC_BOOST_TEST in config.h for tests only

This define is used by the unit tests; there is no reason why it should
be on the list that must be provided by other projects that use our
headers (the other projects don't use our tests).
Trim unnecessarily required config vars

The modules declare which configuration variables they need to have
present in config.h, not only their own but also defined in projects
using them.

However, a lot of these variables are not actually used in the headers!

This changeset removes all HAVE_XXX variables which is not present in
any opm/*.h* file in these projects, and thus there is no need for the
client to specify.

Note that only the variables used by the module *itself* should be
listed; the build system will setup the complete list from the
prerequisites.
Set standard variables for package after probing

Standard variables must be set in order for the package to be picked
up with the OpmFind/OpmPackage dance.
OPM-Material module must have DUNE Common and DUNE ISTL
Only search for a package again if not found

If it is already found, most packages sets a flag that makes it exit
immediately anyway. We usually cannot rerun find_package to get another
combinations of modules. Here we check the flag before we start looking
for the package to avoid the endless stream of messages.
Propagate REQUIRED and QUIET according to parent level

If an optional package require further packages, those packages should
not be marked as REQUIRED because CMake will then terminate the
configuration if it is not found (although it is transitively optional).

Conversely, we should be able to specify REQUIRED prerequisites for
REQUIRED packages and have a failure cascade up to termination.

This changeset allows us to specify REQUIRED or not in the list of
prerequisites, and have OpmPackage sort of whether it should be honored
or not.
Allow found-variable to be named as package

Boost does not set the BOOST_FOUND variable to indicate that the
package is found (as it would if it had used the usual routine
find_package_handle_standard_args), but rather Boost_FOUND. This
patch tests for that condition, and print some more helpful debug
messages if a prerequisite is missing.