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
@@ -626,7 +626,15 @@ endif()")
 
   # install pkg-config files
   if(EXISTS ${PROJECT_SOURCE_DIR}/${ProjectName}.pc.in)
-    create_and_install_pkconfig(${DUNE_INSTALL_LIBDIR})
+    # Make sure that CMake files are installed to DATAROOTDIR
+    # for header only libraries
+    if(_has_lib)
+      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_PKGCONFIGDIR})
   endif()
 
   ###########################
