Propagate OPUS_VERSION to config.h.

I'm not sure how this worked before, the the previous version
string fiddling commit ended up not defining OPUS_VERSION in
the autoconf build, so opus_get_version_string() returned
'unknown'.

configure.ac

@@ -46,6 +46,8 @@ VERSION=$OPUS_VERSION
 
 # For config.h.
 AC_SUBST(OPUS_VERSION)
+AC_DEFINE_UNQUOTED([OPUS_VERSION], ["$OPUS_VERSION"],
+                   [Opus library version string])
 
 # For libtool.
 dnl Please update these for releases.