https://bugs.gentoo.org/867283
https://wiki.gentoo.org/wiki/Musl_porting_notes#cdefs.h:_No_such_file_or_directory
--- a/pico-sdk/src/host/pico_platform/include/pico/platform.h
+++ b/pico-sdk/src/host/pico_platform/include/pico/platform.h
@@ -13,7 +13,11 @@
 
 #ifdef __unix__
 
-#include <sys/cdefs.h>
+#if defined __has_include
+# if __has_include (<sys/cdefs.h>)
+#  include <sys/cdefs.h>
+# endif
+#endif
 
 #endif