Detect out of date flex in MSVC builds.
Detect out of date flex in MSVC builds.

Per recent discussion, following a report from Quan Zongliang.
The same logic is used as in pgbison.pl.

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