It's not set in the meson files and it's not done in autotools builds.
@ -501,11 +501,6 @@
/* version revision number */
#mesondefine PACKAGE_VERSION_REVISION
/* Same as _REENTANT for some other OSes. */
#ifndef _THREAD_SAFE
#mesondefine _THREAD_SAFE
#endif
/* Enable GNU extensions on systems that have them. */
#ifndef _GNU_SOURCE
#mesondefine _GNU_SOURCE