The sources previously hard-coded the location of the default Jami
ringtone as "/usr/share/ring/ringtones/default.opus".  It should
instead be computed from the already existing PROGSHAREDIR macro,
which is built from Autoconf's configurable datadir.

* globals.mk (-DPROGSHAREDIR): Rename the 'ring' subdirectory to
'jami'.
* meson.build (conf): Likewise.
* ringtones/Makefile.am (dist_jamiringtones_DATA): s/ring/jami/g
* ringtones/meson.build: Likewise.
* src/account.cpp (DEFAULT_RINGTONE_PATH): Define in terms of
PROGSHAREDIR.
* bin/dbus/Makefile.am (libclient_dbus_la_CXXFLAGS): Remove extraneous
flags, already propagated via AM_CPPFLAGS.  Honor AM_CXXFLAGS.
* src/client/Makefile.am (libclient_la_CXXFLAGS): Remove.
* src/plugin/Makefile.am (libplugin_la_CXXFLAGS): Likewise.

Change-Id: Icaa0a92e896f86b0bb07f82e4079570db08fafbd