Nanopb generator attempts to calculate the maximum encoded size of messages
when possible. If the message size is unbounded due to e.g. callbacks, the
define for it will be skipped. However, for oneofs we need to calculate the
maximum of all fields inside it, which is not easily done in C in compile time.
The current method uses sizeof(union { .. }) to calculate the largest value.
A recent change in 3ff6743173 (#415, #494) moved the union to a separate line.
This in turn caused compilation errors when the union was defined but one of
the symbols it depends on (from another .proto file) was not defined.
This commit fixes it by adding #if defined() guard around the definition.
Petteri Aimonen
ca01b338e9