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>