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
@@ -520,6 +520,17 @@ endif()
     ${CONFIG_VERSION_FILE}
     ${PROJECT_BINARY_DIR}/${ProjectName}-config-version.cmake @ONLY)
 
+  # 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_LIBDIR})
+  create_and_install_pkconfig(${DUNE_INSTALL_PKGCONFIGDIR})
+
   ###########################
   ### CREATE TARGETS FILE ###
   ###########################
