Add CMake exported targets

[jamiesnape]

Makes it easier to use OctoMap in other CMake projects (and avoids issues like #138).

[ahornung]

@jamiesnape Could you quickly outline what this means for libs depending on CMake, how they can or must use it?

Is this a (potentially) breaking change that needs to go into a new Minor version, or is a new Patch release sufficient?

[jamiesnape]

It is not a breaking change. Projects can continue to use the current logic, so a patch release is fine.

As well as:

find_package(octomap)
if(octomap_FOUND)
  add_executable(foo foo.c)
  target_include_directories(foo ${OCTOMAP_INCLUDE_DIRS})
  target_link_libraries(foo ${OCTOMAP_LIBRARIES})
endif()

you could also use:

find_package(octomap)
if(octomap_FOUND)
  add_executable(bar bar.c)
  target_link_libraries(bar octomap)
endif()

or to link with the static library:

find_package(octomap)
if(octomap_FOUND)
  add_executable(foobar foobar.c)
  target_link_libraries(foobar octomap-static)
endif()

(no call to include_directories() or target_include_directories() required, nor is octomath required to be added to the target_link_libraries() call.)

This would also be correct:

if(TARGET octomap-static)
  add_executable(foobar foobar.c)
  target_link_libraries(foobar octomap-static)
endif()

[ahornung]

Cool, thanks!