Cannot perform static_assert in C++ on header files generated by pioasm

[rhulme]

Because the pio_program structure is not declared constexpr, it's not possible to perform compile-time checks on the contents.

In my project, I have a couple of PIO programs that I would like to ensure both fit in a single PIO instance (i.e. in the 32-word memory) and discover at compile-time if changes are made that would prevent this, rather than with a panic at run-time.

Obviously, constexpr is a C++ keyword so can't be used if the header file is included from a plain C file, but the keyword to use can be determined via checking whether __cplusplus is defined.