build: respect '--localedir' from configure.

Makefile shouldn't overwrite the 'localedir' variable, because that's
what '--localedir' option in 'configure' is for.
diff --git a/src/Makefile.am b/src/Makefile.am
index f7d0e09..12b2327 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -1,5 +1,4 @@
 AM_YFLAGS = -d
-localedir = $(datadir)/locale
 AM_CPPFLAGS = -DLOCALEDIR=\"$(localedir)\"
 LIBS = @LIBS@