Header and library directory may be specified separately by using the
variables e.g. OPM_CORE_INCLUDE_DIR and OPM_CORE_LIB_DIR. These override
even the OPM_CORE_ROOT. This allows us to use a backported version which
uses versioned directory names.
v2: incorporate Roland's nitpicks (I hope)
v3: It was decided that it's better to change the default behavior
v4: make sure the CMake syntax is correct even if the value of the
variable contains spaces (if it contains double quotes it's a
different matter, but that's pretty much a corner case, IMHO)
v5: properly escape backslash and double quote characters in the cmake
syntax
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.
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.
