Rename target to DDRec, this required changes in many other CMakeLists to be consistent

Re-arrange Gear dependent parts into single if scope
Add SYSTEM to GEAR IncludeDirs

Use variables to define libraries in one place and add optional ones (i.e., gear) to the list

Create plugin_sources variable to create a DDRecPlugin independent of the DDRec library parts

No Whitespace after and before parenthesis
Assume ROOT_INCLUDE_DIRS is added in main CMakeLists file
Comment Setting of CMAKE_CXX_FLAGS