compilersupport: fix build with GCC < 11

Commit 45e4641 ("Fix build with GCC < 11: [[fallthrough]] is supported
but not allowed in C") introduced a fix for old GCC versions, but commit
91d1c50 ("compilersupport: fix compilation in C23 mode") reverted it,
reintroducing the build failure.

Revert the revert.

Signed-off-by: Florian Larysch <fl@n621.de>