build: Conditionalize bsd_getopt() on macOS

The system library provides a getopt() with BSD semantics.

configure.ac

@@ -309,6 +309,7 @@ AC_CHECK_FUNCS([\
 ])
 
 need_arc4random=yes
+need_bsd_getopt=yes
 need_md5=yes
 need_nlist=yes
 need_strl=yes
@@ -330,6 +331,7 @@ AS_CASE([$host_os],
     # On macOS these are provided by the system, and libbsd has never built
     # there, so we can avoid providing these with no ABI breakage.
     need_arc4random=no
+    need_bsd_getopt=no
     need_transparent_libmd=no
     need_md5=no
     need_nlist=no
@@ -345,6 +347,7 @@ AS_CASE([$host_os],
 AM_CONDITIONAL([HAVE_GETENTROPY], [test "x$ac_cv_func_getentropy" = "xyes"])
 
 AM_CONDITIONAL([NEED_ARC4RANDOM], [test "x$need_arc4random" = "xyes"])
+AM_CONDITIONAL([NEED_BSD_GETOPT], [test "x$need_bsd_getopt" = "xyes"])
 AM_CONDITIONAL([NEED_TRANSPARENT_LIBMD], [test "x$need_transparent_libmd" = "xyes"])
 AM_CONDITIONAL([NEED_MD5], [test "x$need_md5" = "xyes"])
 AM_CONDITIONAL([NEED_NLIST], [test "x$need_nlist" = "xyes"])

include/bsd/unistd.h

@@ -47,6 +47,7 @@
 #endif
 
 __BEGIN_DECLS
+#if !defined(__APPLE__)
 extern int optreset;
 
 #ifdef LIBBSD_OVERLAY
@@ -55,6 +56,7 @@ extern int optreset;
 #endif
 
 int bsd_getopt(int argc, char * const argv[], const char *shortopts);
+#endif
 
 mode_t getmode(const void *set, mode_t mode);
 void *setmode(const char *mode_str);

src/Makefile.am

@@ -75,7 +75,6 @@ EXTRA_libbsd_la_DEPENDENCIES += \
 	# EOL
 endif
 libbsd_la_SOURCES = \
-	bsd_getopt.c \
 	closefrom.c \
 	dehumanize_number.c \
 	err.c \
@@ -116,6 +115,12 @@ libbsd_la_SOURCES = \
 	wcslcpy.c \
 	# EOL
 
+if NEED_BSD_GETOPT
+libbsd_la_SOURCES += \
+	bsd_getopt.c \
+	# EOL
+endif
+
 if NEED_ARC4RANDOM
 if !HAVE_GETENTROPY
 libbsd_la_SOURCES += \