Some fixes and simplifications of gazebo_plugins/CMakeLists.txt

The changes are easier to view in the individual commits.

e02bb43:  Add dependencies on dynamic reconfigure files
Occasionally the build can fail due to some targets having an
undeclared dependency on automatically generated dynamic
reconfigure files (GazeboRosCameraConfig.h for example). This
commit declares several of those dependencies.

af973a9: Put some cmake lists on multiple lines to improve readability.

f7effe9: Simplify gazebo_plugins/CMakeLists.txt
Replace cxx_flags and ld_flags variables with simpler cmake macros
and eliminate unnecessary references to SDFormat_LIBRARIES, since
they are already part of GAZEBO_LIBRARIES.