conf: Install charon-specific snippets also when charon itself is not built

To make the default strongswan.conf, with `load_modular` enabled, work
if charon itself is not built, we enable generating the charon-specific
snippets also for the two other daemons that fall back on reading
options from the `charon` section.

conf/Makefile.am

@@ -117,7 +117,7 @@ confsnippets = $(alloptions:opt=conf)
 plugins_install_tmp = $(config_plugins:%=plugins/%.tmp)
 plugins_install_src = $(config_plugins:%=plugins/%.conf)
 
-if USE_CHARON
+if USE_CHARON_CONF
 charon_install_src = $(charon_plugins:%=plugins/%.conf)
 endif
 if USE_CMD