At least as of Postgres 10 beta, you can look for configure to fail. If you have not given permission for the strong-random to be omitted (via a configure setting), then Postgres built without strong-random will fail during configure.
To quote the 10 beta doc:
• Add more robust random number generators to be used for cryptographic secure uses (Magnus Hagander, Michael Paquier, Heikki Linnakangas)
If no strong random number generator can be found, configure will fail unless the configure --disable-strong-random is used. However, with this option, pgcrypto functions requiring a strong random number generator will be disabled.