Replace os_get_ltrace_conf_filename with os_get_ltrace_conf_filenames

- I.e. allow returning a list of filenames, as opposed to a single
  home directory name.  This is to implement exactly the legacy config
  file behavior on Linux.
diff --git a/configure.ac b/configure.ac
index 16c7a61..ae6bb14 100644
--- a/configure.ac
+++ b/configure.ac
@@ -226,6 +226,7 @@
 
 AM_CPPFLAGS=" \
 	-DSYSCONFDIR="'\"$(sysconfdir)\"'" \
+	-DPKGDATADIR="'\"$(pkgdatadir)\"'" \
 	${AM_CPPFLAGS} \
 	-I\$(top_srcdir)/sysdeps/${HOST_OS}/${HOST_CPU} \
 	-I\$(top_srcdir)/sysdeps/${HOST_OS} \