Tests use POSIX extensions, but default standard is ISO C 99
The file tests/atk_test_util.c
uses timespec
struct: https://gitlab.gnome.org/GNOME/at-spi2-atk/blob/b2356a1e1069ced3239657bc7251555838f1162d/tests/atk_test_util.c#L75. This is a POSIX extension (see, e.g., https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/time.h.html) and requires for example -std=gnu99
to be compiled, but the default standard of the library is c99
.