[Release][CMake] Add dune_target_enable_all_packages()

This patch adds a new function dune_target_enable_all_packages() that
works like dune_enable_all_packages(), but only acts on the given list
of targets.

This function is mostly needed to be able to use the new
dune_enable_all_packages() feature in a DUNE module that defines its own
libraries. Specifically, the "nice" default way of handling those
libraries only works on CMake 3.1+. To be compatible with older CMake
versions, the library has to be manually added in the top-level
CMakeLists.txt file before the call to dune_enable_all_packages(). But
the library also needs to get all the package flags from the DUNE
modules the current one depends on, which can be done using the new
function added by this patch.

cmake/modules/DuneEnableAllPackages.cmake

@@ -32,11 +32,24 @@
 # Warning: The library feature requires CMake 3.1+. If you use the feature with older versions, CMake
 #          will emit a fatal error. Moreover, it will issue a warning if the cmake_minimum_required()
 #          version is older than 3.1.
+# Note:    If you want to use dune_enable_all_packages() with an older version of CMake and your DUNE mdule
+#          creates its own library, you have to manually create the library in the top-level CMakeLists.txt
+#          file using dune_add_library() (with all sources listed within that call), use
+#          dune_target_enable_all_packages() to add all packages to the library and finally list that library
+#          under LIBRARIES in the call to dune_register_package_flags(). See dune-pdelab for an example of
+#          how to do this correctly.
 #
 # For a description of the APPEND option, see the documentation of dune_register_package_flags().
 # With the VERBOSE option set, the list of flags is printed during configure.
 #
 #
+# dune_target_enable_all_packages(TARGETS [target] ...)
+#
+# Adds all currently registered package flags (see dune_register_package_flags()) to the given targets.
+# This macro is mainly intended to help write DUNE modules that want to use dune_enable_all_packages() and
+# define their own libraries, but need to be compatible with CMake < 3.1.
+#
+#
 # dune_register_package_flags(COMPILE_DEFINITIONS [flags]
 #                             INCLUDE_DIRS {includes]
 #                             LIBRARIES [libs]
@@ -190,6 +203,22 @@ Update the cmake_minimum_required() call in your main CMakeLists.txt file to get
 endfunction(dune_enable_all_packages)
 
 
+function(dune_target_enable_all_packages)
+  foreach(_target ${ARGN})
+
+    get_property(all_incs GLOBAL PROPERTY ALL_PKG_INCS)
+    target_include_directories(${_target} PUBLIC ${all_incs})
+
+    get_property(all_defs GLOBAL PROPERTY ALL_PKG_DEFS)
+    target_compile_definitions(${_target} PUBLIC ${all_defs})
+
+    get_property(all_libs GLOBAL PROPERTY ALL_PKG_LIBS)
+    target_link_libraries(${_target} PUBLIC ${DUNE_LIBS} ${all_libs})
+
+  endforeach()
+endfunction(dune_target_enable_all_packages)
+
+
 function(dune_library_add_sources lib)
 
   # This only works for CMAKE 3.1+ because target_sources() - which we use to add sources to the