Makefile tweaks for helloworld template plugin

 - Do not force use of clang++ to build the plugin (broken on recent debian
   and derivates: https://bugs.debian.org/643959)
 - Do not force clean on install
 - Use -fPIC to build objects (or g++ complains)
Merge pull request #2271 from strk/2.3.x-helloworld-makefile

2.3.x helloworld makefile tweaks