Inconsistent use of diagnostic pragmas (`-Wreturn-type`)

[hirrolot]

Currently, inside match99, we emit the ... diagnostic push and ... diagnostic ignored "-Wreturn-type" pragmas, but without otherwise99 we won't return to the previous diagnostics state:

datatype99/datatype99.h

Line 154 in 344206e

DATATYPE99_PRIV_SUPPRESS_W_RETURN_TYPE \

I've spent some time trying to figure out how to automatically resolve this warning but with no success; therefore, the best solution for now is just don't suppress -Wreturn-type but to force a user to suppress it, if necessary.

FYI: The -Wreturn-type warning can be emitted due to the following reasons:

• A false positive due to the generation of single-step for-loops and the switch statement over a (conceptually) strict enumeration; see this SO question.
• C enumerations are seen by a compiler as ordinary integers -- see another SO question of mine.