[cmake] Update build system

The creation of the library needs to be updated, because
otherwise the library is not found by downstream modules.

CMakeLists.txt

-cmake_minimum_required(VERSION 2.8.6)
+if(dune-common_VERSION VERSION_GREATER_EQUAL 2.10.0)
+    cmake_minimum_required(VERSION 3.16)
+else()
+    cmake_minimum_required(VERSION 3.13)
+endif()
 project(dune-solvers CXX)
 
 if(NOT (dune-common_DIR OR dune-common_ROOT OR
@@ -22,9 +26,17 @@ dune_project()
 find_package(SuiteSparse OPTIONAL_COMPONENTS UMFPACK)
 include(AddSuiteSparseFlags)
 
+# Create library target and export it as Dune::Solvers
+dune_add_library(dunesolvers EXPORT_NAME Solvers LINK_LIBRARIES ${DUNE_LIBS})
+
+dune_register_package_flags(LIBRARIES dunesolvers)
+
 add_subdirectory("dune")
 add_subdirectory("doc")
 add_subdirectory("cmake/modules")
 
-# finalize the dune project, e.g. generating config.h etc.
-finalize_dune_project(GENERATE_CONFIG_H_CMAKE)
+if(dune-common_VERSION VERSION_GREATER_EQUAL 2.10.0)
+    finalize_dune_project()
+else()
+    finalize_dune_project(GENERATE_CONFIG_H_CMAKE)
+endif()

dune/solvers/CMakeLists.txt

-dune_add_library("dunesolvers"
-    iterationsteps/blockgssteps.cc
-    solvers/criterion.cc)
-
-dune_register_package_flags(LIBRARIES dunesolvers)
-
 add_subdirectory("common")
 add_subdirectory("iterationsteps")
 add_subdirectory("norms")
@@ -12,6 +6,10 @@ add_subdirectory("solvers")
 add_subdirectory("test")
 add_subdirectory("transferoperators")
 
+target_sources(dunesolvers PRIVATE
+    iterationsteps/blockgssteps.cc
+    solvers/criterion.cc)
+
 install(FILES
     computeenergy.hh
     DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/dune/solvers)