simple_listener: minor, warning removal

libsndfile API: expected 'const int *' but argument is of type 'uint32_t *'
jack_listener: minor, warning removal

format '%s' expects a matching 'char *' argument
jack_listener: minor, warning removal

control reaches end of non-void function
Minor: update .gitignore
Build, minor: update warning option "-W" to new name "-Wextra"

From gcc documentation:
"The older name is still supported, but the newer name is more descriptive."
Examples makefiles: harmonization and cleaning.

Remove some unneeded linker flags, add some missing headers dependencies,
same optimization and debug options for all.
Endpoints cleaning: warning removal for "unused parameter"