tag: id3v2: add id3v2 tag parsing helpers

https://bugzilla.gnome.org/show_bug.cgi?id=654388
diff --git a/configure.ac b/configure.ac
index f14abe1..a76691f 100644
--- a/configure.ac
+++ b/configure.ac
@@ -505,6 +505,17 @@
   AM_CONDITIONAL(USE_ISO_CODES, false)
 fi
 
+dnl *** zlib is optionally used by id3 tag parsing in libgsttag ***
+translit(dnm, m, l) AM_CONDITIONAL(USE_ZLIB, true)
+AG_GST_CHECK_FEATURE(ZLIB, [zlib support for ID3 parsing in libgsttag],, [
+  AG_GST_CHECK_LIBHEADER(ZLIB,
+    z, uncompress,, zlib.h, [
+    HAVE_ZLIB="yes"
+    ZLIB_LIBS="-lz"
+    AC_SUBST(ZLIB_LIBS)
+  ])
+])
+
 dnl *** sys plug-ins ***
 
 echo