MQTT library no longer works

The MQTT library seems to only work for building with firmware level 0.4.5. Anything after that, you get a bunch of errors like this.

../hal/src/stm32f2xx/deepsleep_hal_impl.h:23:60: error: section attribute not allowed for '<anonymous>'
 #define retained  __attribute__((section(".retained_user")))

I’d like to use MQTT for both the Photon and the Electron. Is there another MQTT implementation people are using?

Hmm, what code are you building.
If I try the samples provided with the library, both build just fine (only mqttqostest.ino needs a blank line added at the top) for Photon 0.4.9