Fix usage of wrong version variable

Author: alcaeus

config.m4

@@ -255,7 +255,7 @@ if test "$PHP_MONGODB" != "no"; then
 
     PKG_CHECK_MODULES([PHP_MONGODB_MONGOC], [mongoc2 >= 2.0.1], [
       PHP_MONGODB_MONGOC_VERSION=`$PKG_CONFIG mongoc2 --modversion`
-      PHP_MONGODB_MONGOC_VERSION_STRING="System ($PHP_MONGODB_BSON_VERSION)"
+      PHP_MONGODB_MONGOC_VERSION_STRING="System ($PHP_MONGODB_MONGOC_VERSION)"
 
       PHP_MONGODB_CFLAGS="$PHP_MONGODB_CFLAGS $PHP_MONGODB_MONGOC_CFLAGS"
       PHP_EVAL_LIBLINE($PHP_MONGODB_MONGOC_LIBS, MONGODB_SHARED_LIBADD)