WIP: playground for config.h replacement

This MR is intended for experimenting with ideas in #234 (closed) and not a direct candidate for merge.

cmake/modules/DuneInstance.cmake

@@ -106,8 +106,6 @@
 #
 #      // @GENERATED_SOURCE@
 #
-#      #include <config.h>
-#
 #      #include <mytest.hh>
 #
 #      int main() {
@@ -134,8 +132,6 @@
 #
 #      // @GENERATED_SOURCE@
 #
-#      #include <config.h>
-#
 #      #include <mytest.hh>
 #
 #      template void MyTestSuite::test<@TYPE@>();