engine: platform: linux: define portability macro for sigevent (oh, glibc...)

engine/platform/linux/sys_linux.c

@@ -29,7 +29,8 @@ GNU General Public License for more details.
 #include "platform/platform.h"
 #include <sys/types.h>
 
-#if defined(__GLIBC__) && (__GLIBC__ <= 2) && (__GLIBC_MINOR__ <= 30)
+#if defined( __GLIBC__ )
+#if ( __GLIBC__ <= 2 ) && ( __GLIBC_MINOR__ <= 30 )
 // Library support was added in glibc 2.30.
 // Earlier glibc versions did not provide a wrapper for this system call,
 // necessitating the use of syscall(2).
@@ -39,7 +40,14 @@ static pid_t gettid( void )
 {
 	return syscall( SYS_gettid );
 }
-#endif
+#endif // ( __GLIBC__ <= 2 ) && ( __GLIBC_MINOR__ <= 30 )
+
+// Glibc misses this macro in POSIX headers (bits/types/sigevent_t.h)
+// but it does present in Linux headers (asm-generic/siginfo.h) and musl
+#if !defined( sigev_notify_thread_id )
+#define sigev_notify_thread_id _sigev_un._tid
+#endif // !defined( sigev_notify_thread_id )
+#endif // defined(__GLIBC__)
 
 static void *g_hsystemd;
 static int (*g_pfn_sd_notify)( int unset_environment, const char *state );