add empty file as needed for add_plugin
add tests only if lib was found
fix journald plugin (include+lib)
c++ plugins converted to add_plugin

improves build time (clean build with all plugins) from
make -j 4  214,13s user 7,85s system 378% cpu 58,653 total
to
make -j 4  206,08s user 7,60s system 381% cpu 56,080 total

convert struct+type plugins
add_cpp_plugintest honors MEMLEAK and BUILD_TESTING
add_cppheaders only add *.hpp (and not CMakeLists.txt)
add CPP option for add_plugin
fix ERROR -> SEND_ERROR
shared libraries use objects library too