libpurple: Remove PURPLE_PLUGINS define from purple.h
The PURPLE_PLUGINS definition in purple.h is no longer needed now
that nothing uses it. This patch removes it, the PLUGINS_DEFINE
configuration data which was used to add it, and moves the
purple_h_conf configuration data variable into libpurple/meson.build
as nothing else in the root meson.build file uses it.
meson: Update plugins option to reflect what it now does
Previously, the 'plugins' option would toggle libpurple plugin
support. Now that libpurple's plugin support is required, the
'plugins' option instead merely toggles building plugins
(including prpls). Ultimately we probably want an option for
toggling the building of plugins and a separate one for prpls,
but that can come later. This patch updates the description of
the 'plugins' option to better reflect what it now does.