As long as you don't return structs, as the compiler may introduce memcpys here. C doesn't have C++'s RVO, last time I checked many cases that GCC and Clang did optimize in C++ weren't optimized in C.
That works, at the expense of possibly hiding bugs in plain sight. Macros are a double edged sword in that sense, you can almost but not quite create a DSL on top of C that is a lot more safe but it has its drawbacks, and you need to be very good at mentally modeling macro expansion to read and debug code like that in order to ensure that it does what it seems to be saying that it will do.
But yes, sadly I think it's still generally advisable to avoid returning large structs by value.
Note that this has nothing to do with RVO; as I understand it, RVO is about eliding copy constructors, not actual memory copies, and c has no copy constructors and so gets 'RVO' in all cases.
For the use cases that are too expensive to use functions, macros can be used instead.
However that is only if C++ cannot be used at all, otherwise don't bother.