build: Use macOS' compatibility macros to enable all features.
* configure.ac: On macOS, use the compatibility macros to expose every feature of the libc. This is the equivalent of _GNU_SOURCE on GNU libc.
Not defining this leads to compilation errors or superfluous warnings
on macOS.
- GnuPG-bug-id: T2910
- Signed-off-by: Justus Winter <justus@g10code.com>