Meson: Fix SYSCONFDIR definition.

SYSCONFDIR was set to the wrong path if prefix is not '/usr' and
sysconfdir is an absolute path. Use join_paths() to fix it.

Also remove the special case for prefix '/usr'. In that case Meson
already sets sysconfdir to the absolute path '/etc', so just using
join_paths() will return the correct value.

    join_paths('/usr/local', 'etc') => '/usr/local/etc'
    join_paths('/usr/local', '/etc') => '/etc'
    join_paths('/usr', '/etc') => '/etc'

meson.build

@@ -108,13 +108,7 @@ if scdoc.found()
 	endforeach
 endif
 
-# If prefix is '/usr', sysconfdir will be explicitly set to '/etc' by Meson to
+add_project_arguments('-DSYSCONFDIR="/@0@"'.format(join_paths(prefix, sysconfdir)), language : 'c')
-# 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 != ''
@@ -159,14 +153,7 @@ subdir('swaylock')
 config = configuration_data()
 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,