Workaround a GCC 5 constexpr function bug

al/auxeffectslot.cpp

@@ -230,7 +230,12 @@ constexpr EffectSlotType EffectSlotTypeFromEnum(ALenum type)
     case AL_EFFECT_DEDICATED_DIALOGUE: return EffectSlotType::DedicatedDialog;
     case AL_EFFECT_CONVOLUTION_REVERB_SOFT: return EffectSlotType::Convolution;
     }
+    /* GCC 5 doesn't like this in a constexpr function. Clang reports itself as
+     * GCC 4 or 5, so explicitly allow that.
+     */
+#if !defined(__GNUC__) || (__GNUC__ > 5) || defined(__clang__)
     ERR("Unhandled effect enum: 0x%04x\n", type);
+#endif
     return EffectSlotType::None;
 }