cmake: do not quote variables for config.h anymore

v2: incorporate Roland's nitpicks (I hope)
v3: It was decided that it's better to change the default behavior
v4: make sure the CMake syntax is correct even if the value of the
    variable contains spaces (if it contains double quotes it's a
    different matter, but that's pretty much a corner case, IMHO)
v5: properly escape backslash and double quote characters in the cmake
    syntax

cmake/Modules/FindCXX11Features.cmake

@@ -74,6 +74,13 @@ CHECK_CXX_SOURCE_COMPILES("
 "  HAVE_SHARED_PTR
 )
 
+# this is required by dune-common to avoid linker errors. "fun"!
+if (HAVE_SHARED_PTR)
+  set(HAVE_MAKE_SHARED 1)
+  set(SHARED_PTR_HEADER "<memory>")
+  set(SHARED_PTR_NAMESPACE "std")
+endif()
+
 # nullptr
 CHECK_CXX_SOURCE_COMPILES("
     #include <memory>