Remove broken PGXS code for pg_xlogdump
Remove broken PGXS code for pg_xlogdump

With the PGXS boilerplate in place, pg_xlogdump currently fails with an
ominous error message that certain targets cannot be built because
certain files do not exist.  Remove that and instead throw a quick error
message alerting the user of the actual problem, which should be easier
to diagnose that the statu quo.

Andres Freund

lisakowen/gpdb,xinzweb/gpdb,greenplum-db/gpdb,oberstet/postgres-xl,ashwinstar/gpdb,pavanvd/postgres-xl,greenplum-db/gpdb,lisakowen/gpdb,Postgres-XL/Postgres-XL,zeroae/postgres-xl,ashwinstar/gpdb,zeroae/postgres-xl,ashwinstar/gpdb,pavanvd/postgres-xl,xinzweb/gpdb,zeroae/postgres-xl,ovr/postgres-xl,xinzweb/gpdb,50wu/gpdb,adam8157/gpdb,yazun/postgres-xl,Postgres-XL/Postgres-XL,zeroae/postgres-xl,techdragon/Postgres-XL,lisakowen/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,adam8157/gpdb,oberstet/postgres-xl,yazun/postgres-xl,adam8157/gpdb,jmcatamney/gpdb,lisakowen/gpdb,yazun/postgres-xl,adam8157/gpdb,oberstet/postgres-xl,techdragon/Postgres-XL,ashwinstar/gpdb,xinzweb/gpdb,jmcatamney/gpdb,xinzweb/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,oberstet/postgres-xl,adam8157/gpdb,ovr/postgres-xl,adam8157/gpdb,ashwinstar/gpdb,50wu/gpdb,jmcatamney/gpdb,ovr/postgres-xl,greenplum-db/gpdb,pavanvd/postgres-xl,jmcatamney/gpdb,lisakowen/gpdb,50wu/gpdb,lisakowen/gpdb,pavanvd/postgres-xl,50wu/gpdb,zeroae/postgres-xl,adam8157/gpdb,pavanvd/postgres-xl,oberstet/postgres-xl,Postgres-XL/Postgres-XL,techdragon/Postgres-XL,lisakowen/gpdb,Postgres-XL/Postgres-XL,50wu/gpdb,50wu/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,xinzweb/gpdb,lisakowen/gpdb,xinzweb/gpdb,yazun/postgres-xl,Postgres-XL/Postgres-XL,ovr/postgres-xl,xinzweb/gpdb,50wu/gpdb,techdragon/Postgres-XL,adam8157/gpdb,techdragon/Postgres-XL,ovr/postgres-xl,greenplum-db/gpdb,greenplum-db/gpdb,50wu/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,yazun/postgres-xl