[glib] Restore gl_GLIBC21 to configure; needed for libcharset

[John Ralls <jralls src gnome org>]

commit 5aead642c29d2fcd4cc4152449eb434b52edc6af
Author: John Ralls <jralls ceridwen us>
Date:   Fri Oct 4 11:45:47 2013 -0700

    Restore gl_GLIBC21 to configure; needed for libcharset

 configure.ac |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 9e55ff5..c6391af 100644
--- a/configure.ac
+++ b/configure.ac
@@ -878,6 +878,7 @@ AC_STRUCT_DIRENT_D_TYPE
 AM_LANGINFO_CODESET
 AC_CHECK_HEADERS([stddef.h stdlib.h string.h])
 AC_CHECK_FUNCS(setlocale)
+gl_GLIBC21
 
 # check additional type sizes
 AC_CHECK_SIZEOF(size_t)