Improve make help

[vornkat-iis]

The current output for make help looks like this:

The following are some of the valid targets for this Makefile:
... all (the default if no target is provided)
... clean
... depend
... edit_cache
... install
... install/local
... install/strip
... list_install_components
... rebuild_cache
... coverage
... doc
... run_test
... gtest
... gtest_main
... japi
... japi-static
... japi_objs
... testsuite
... src/creadline.o
... src/creadline.i
... src/creadline.s
... src/japi.o
... src/japi.i
... src/japi.s
... src/japi_pushsrv.o
... src/japi_pushsrv.i
... src/japi_pushsrv.s
... src/japi_utils.o
... src/japi_utils.i
... src/japi_utils.s
... src/networking.o
... src/networking.i
... src/networking.s
... src/rw_n.o
... src/rw_n.i
... src/rw_n.s
... test/japi_test.o
... test/japi_test.i
... test/japi_test.s

Reduce to actual targets and add descriptions