Add -lm to SHLIB_LINK for contrib/btree_gist.
Add -lm to SHLIB_LINK for contrib/btree_gist.

Now that btree_gist contains a reference to isinf(), this is necessary
at least on some platforms.  Per buildfarm.

Postgres-XL/Postgres-XL,greenplum-db/gpdb,adam8157/gpdb,tpostgres-projects/tPostgres,pavanvd/postgres-xl,pavanvd/postgres-xl,postmind-net/postgres-xl,xinzweb/gpdb,50wu/gpdb,xinzweb/gpdb,jmcatamney/gpdb,Postgres-XL/Postgres-XL,kmjungersen/PostgresXL,adam8157/gpdb,snaga/postgres-xl,50wu/gpdb,xinzweb/gpdb,arcivanov/postgres-xl,arcivanov/postgres-xl,pavanvd/postgres-xl,arcivanov/postgres-xl,techdragon/Postgres-XL,techdragon/Postgres-XL,greenplum-db/gpdb,xinzweb/gpdb,Postgres-XL/Postgres-XL,jmcatamney/gpdb,kmjungersen/PostgresXL,xinzweb/gpdb,snaga/postgres-xl,ashwinstar/gpdb,greenplum-db/gpdb,arcivanov/postgres-xl,oberstet/postgres-xl,adam8157/gpdb,ashwinstar/gpdb,yazun/postgres-xl,kmjungersen/PostgresXL,oberstet/postgres-xl,yazun/postgres-xl,pavanvd/postgres-xl,ovr/postgres-xl,50wu/gpdb,snaga/postgres-xl,xinzweb/gpdb,Postgres-XL/Postgres-XL,jmcatamney/gpdb,xinzweb/gpdb,greenplum-db/gpdb,50wu/gpdb,adam8157/gpdb,oberstet/postgres-xl,zeroae/postgres-xl,tpostgres-projects/tPostgres,50wu/gpdb,lisakowen/gpdb,ovr/postgres-xl,zeroae/postgres-xl,tpostgres-projects/tPostgres,postmind-net/postgres-xl,lisakowen/gpdb,zeroae/postgres-xl,postmind-net/postgres-xl,snaga/postgres-xl,jmcatamney/gpdb,arcivanov/postgres-xl,oberstet/postgres-xl,50wu/gpdb,techdragon/Postgres-XL,adam8157/gpdb,50wu/gpdb,lisakowen/gpdb,greenplum-db/gpdb,adam8157/gpdb,snaga/postgres-xl,postmind-net/postgres-xl,ashwinstar/gpdb,lisakowen/gpdb,ashwinstar/gpdb,ovr/postgres-xl,yazun/postgres-xl,greenplum-db/gpdb,adam8157/gpdb,postmind-net/postgres-xl,ovr/postgres-xl,ashwinstar/gpdb,oberstet/postgres-xl,pavanvd/postgres-xl,greenplum-db/gpdb,tpostgres-projects/tPostgres,arcivanov/postgres-xl,kmjungersen/PostgresXL,yazun/postgres-xl,50wu/gpdb,Postgres-XL/Postgres-XL,jmcatamney/gpdb,jmcatamney/gpdb,lisakowen/gpdb,zeroae/postgres-xl,ashwinstar/gpdb,tpostgres-projects/tPostgres,lisakowen/gpdb,jmcatamney/gpdb,yazun/postgres-xl,ashwinstar/gpdb,lisakowen/gpdb,techdragon/Postgres-XL,kmjungersen/PostgresXL,techdragon/Postgres-XL,jmcatamney/gpdb,zeroae/postgres-xl,ovr/postgres-xl,ashwinstar/gpdb,greenplum-db/gpdb,lisakowen/gpdb,adam8157/gpdb,xinzweb/gpdb