build: _DARWIN_C_SOURCE should be 1.

* configure.ac (*-apple-darwin*): Set _DARWIN_C_SOURCE 1.

Cherry-pick master commit of:
b9a14725ec13747dab1d96658b2f7ce09b1ec874

• GnuPG-bug-id: T5440
• Reported-by: Jay Freeman
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>