Set sysconfdir to /etc only if prefix is /usr

PR #2855 basically hardcodes the config file path to /etc, which is a
problem on e.g. FreeBSD, where the expected path for config files of
non-base software is '/usr/local/etc'.
Meson sets sysconfdir to '/etc' explicitly only when prefix is '/usr',
so it is still possible to use '/usr/local' as prefix, and install the
config files under '/usr/local/etc'. This commit allows to do that by
setting sysconfdir based on the value of prefix.

meson.build

@@ -114,7 +114,13 @@ if scdoc.found()
 	endforeach
 endif
 
+# If prefix is '/usr', sysconfdir will be explicitly set to '/etc' by Meson to
+# enforce FHS compliance, so we should look for configs there as well.
+if prefix == '/usr'
 	add_project_arguments('-DSYSCONFDIR="/@0@"'.format(sysconfdir), language : 'c')
+else
+	add_project_arguments('-DSYSCONFDIR="/@0@/@1@"'.format(prefix, sysconfdir), language : 'c')
+endif
 
 version = get_option('sway-version')
 if version != ''
@@ -157,10 +163,17 @@ subdir('swaynag')
 subdir('swaylock')
 
 config = configuration_data()
-config.set('sysconfdir', sysconfdir)
 config.set('datadir', join_paths(prefix, datadir))
 config.set('prefix', prefix)
 
+# If prefix is '/usr', sysconfdir will be explicitly set to '/etc' by Meson to
+# enforce FHS compliance, so we should look for configs there as well.
+if prefix == '/usr'
+	config.set('sysconfdir', sysconfdir)
+else
+	config.set('sysconfdir', join_paths(prefix, sysconfdir))
+endif
+
 configure_file(
 	configuration: config,
 	input: 'config.in',