Install architecture independent pkgconfig file in /usr/share/pkgconfig

This prevents lintian warning pkg-config-unavailable-for-cross-compilation
for the header only modules dune-istl, dune-localfunctions, dune-functions,
and dune-typetree.
--- a/cmake/modules/DuneProject.cmake
+++ b/cmake/modules/DuneProject.cmake
@@ -524,8 +524,13 @@ endif()")
     install(FILES config.h.cmake DESTINATION share/${ProjectName})
   endif()
 
+  if(${ProjectName}_LIBRARIES)
+    set(DUNE_INSTALL_PKGCONFIGDIR ${CMAKE_INSTALL_LIBDIR})
+  else()
+    set(DUNE_INSTALL_PKGCONFIGDIR ${CMAKE_INSTALL_DATAROOTDIR})
+  endif()
   # install pkg-config files
-  create_and_install_pkconfig(${DUNE_INSTALL_LIBDIR})
+  create_and_install_pkconfig(${DUNE_INSTALL_PKGCONFIGDIR})
 
   ###########################
   ### HEADER CONFIG FILEs ###
