Create and register library before add_subdirectory()

Otherwise the tests in the subdirectories will not link to
the lib although tests in other modules do.

dune/solvers/CMakeLists.txt

+dune_add_library("dunesolvers"
+    solvers/criterion.cc)
+
+dune_register_package_flags(LIBRARIES dunesolvers)
+
 add_subdirectory("common")
 add_subdirectory("iterationsteps")
 add_subdirectory("norms")
@@ -6,11 +11,6 @@ add_subdirectory("solvers")
 add_subdirectory("test")
 add_subdirectory("transferoperators")
 
-dune_add_library("dunesolvers"
-    solvers/criterion.cc)
-
-dune_register_package_flags(LIBRARIES)
-
 install(FILES
     computeenergy.hh
     DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/dune/solvers)