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>

Author: florolf

src/compilersupport_p.h

@@ -52,10 +52,14 @@
 #  define cbor_static_assert(x)         ((void)sizeof(char[2*!!(x) - 1]))
 #endif
 
-#if defined(__has_cpp_attribute)    // C23 and C++17
+#if defined(__has_cpp_attribute) && defined(__cplusplus)    // C++17
 #  if __has_cpp_attribute(fallthrough)
 #    define CBOR_FALLTHROUGH            [[fallthrough]]
 #  endif
+#elif defined(__has_c_attribute) && !defined(__cplusplus)   // C23
+#  if __has_c_attribute(fallthrough)
+#    define CBOR_FALLTHROUGH            [[fallthrough]]
+#  endif
 #endif
 #ifndef CBOR_FALLTHROUGH
 #  ifdef __GNUC__