Bug 40193: Add `AT_EMPTY_PATH` definition
AT_EMPTY_PATH
comes with glibc 2.14. However, Debian Wheezy, which we
still use for building our stable Linux bundles, comes only with glibc
2.13 resulting in an "undeclared-identifier"-error during build time.
This problem got introduced by fixing https://bugzilla.mozilla.org/show_bug.cgi?id=1660901.
Closes #40193 (closed).