Backport of function pg_xlog_location_diff() from PostgreSQL 9.2.

There's also a plpgsql function available at

http://vibhorkumar.wordpress.com/2013/02/18/pg_xlog_location_diff-function-for-postgreqsqlppas/

however, this module aims to backport the same function used in PostgreSQL 9.2 in C.

= Installation =

== PostgreSQL Version 9.1 ==

In PostgreSQL 9.1, the CREATE EXTENSION facility should be used. To built
the pg_xlog_location_diff extension module, use the Makefile.extension makefile:

$ USE_PGXS=1 make -f Makefile.extension install

If built successfully, create the extension in your target database, e.g:

$ psql <DBNAME>
=# CREATE EXTENSION pg_xlog_location_diff;

Now the function can be used to calculate the number of bytes between
two given XLOG offsets.

== PostgreSQL Versions below 9.1 ==

PostgreSQL Versions before 9.1 don't have EXTENSION support, instead a contrib
module should be built:

$ USE_PGXS=1 make -f Makefile.contrib install

If built successfully, install the contrib module into your database of wish, e.g.

$ psql -f $(pg_config --sharedir)/contrib/pg_xlog_location_diff <DBNAME>

There's also a file uninstall_pg_xlog_location_diff, which lets you uninstall the function easily.