build: define ABOUT_NLS when configuring

This fixes the various translation issues throughout
Calendar.

https://bugzilla.gnome.org/show_bug.cgi?id=764333

configure.ac

@@ -23,6 +23,9 @@ GETTEXT_PACKAGE=AC_PACKAGE_TARNAME
 AC_SUBST(GETTEXT_PACKAGE)
 AC_DEFINE_UNQUOTED(GETTEXT_PACKAGE,"$GETTEXT_PACKAGE", [GETTEXT package name])
 
+AS_IF([test "x$USE_NLS" != "xno"],
+      [AC_DEFINE([ENABLE_NLS], [1], [Native language support is enabled])])
+
 ## don't rerun to this point if we abort
 AC_CACHE_SAVE