Incorrect usage of an internal Glibc header (sys/cdefs.h)

[xplshn]

/usr/include/sys/cdefs.h:4:2: warning: usage of non-standard #include <sys/cdefs.h> is deprecated [-W#warnings]
    4 | #warning usage of non-standard #include <sys/cdefs.h> is deprecated

Developers like to wrongly include sys/cdefs.h to use the _*_DECLS macros. This is a bug and the correct way to do it is to use:

#ifdef __cplusplus
extern "C" {
#endif

Instead of:

#ifdef __cplusplus
}
#endif

And instead of _END_DECLS

https://wiki.gentoo.org/wiki/Musl_porting_notes

This causes problems on other OSes, like Illumos based ones, such as OpenIndiana, and also when */Linux based distros that use Musl, uClibc, Bionic libc (Android libc) or anything other than glibc try to compile libxo