Search for the prerequisites from package configuration file.

That is the way it is done in other modules, too. We need this
at least for libecl as this target is used in opm-common_LIBRARIES
and not defined if there was no find_package(ecl) call. That call
is now made during execution of find_package_deps(opm-common).

opm-common-prereqs.cmake

@@ -25,3 +25,5 @@ else()
         "Boost 1.44.0
           COMPONENTS system unit_test_framework REQUIRED")
 endif()
+# We the a defined target for libecl
+find_package_deps(opm-common)