Makefile.am: move target clean-local to distclean-local
Makefile.am: move target clean-local to distclean-local

Deleting the pkg-config file and also the *.map and *.sym file in a
clean-local target isn't correct as we have no default automatic way to
regenerate *.map and *.sym files before the build of the libcoap is
starting.

Currently there is no need for such a automatic rebuild, the user
normaly doesn't need to do anything on those files. They even don't
should be rebuild by the users in normal cases.
Adding or removing symbols is a sensible thing and the maintenance is
probably up to the developers. Beside that there is the target
'update-map-file' that is used by the developers to (re)build this two
files if needed.

This commit is also adding a new helper target 'dist-local' with a
depency on 'update-map-file' so we are sure to have actual *.map and
*.sym file if we create a tarball for a release.

Moving the .PHONY targets to the end of the Makefile.am as this is the
typical place.

authmillenon/libcoap,authmillenon/libcoap,authmillenon/libcoap