[core] earlier macOS need define for errno_t (fixes #3107)

Earlier macOS need _DARWIN_C_SOURCE defined for errno_t from sys/types.h

Alternatively, define __STDC_WANT_LIB_EXT1__ >= 1 before include errno.h

x-ref:
  "error: unknown type name 'errno_t' (macOS 10.13.6)"
  https://redmine.lighttpd.net/issues/3107

src/ck.h

@@ -12,6 +12,11 @@
 #ifndef __STDC_WANT_LIB_EXT1__ /*(enable C11 Annex K ext1 *_s functions)*/
 #define __STDC_WANT_LIB_EXT1__ 1
 #endif
+#if defined(__APPLE__) && defined(__MACH__)
+#ifndef _DARWIN_C_SOURCE
+#define _DARWIN_C_SOURCE
+#endif
+#endif
 #include "first.h"
 #ifdef __FreeBSD__
 #ifndef _RSIZE_T_DEFINED /* expecting __EXT1_VISIBLE 1 and _RSIZE_T_DEFINED */

src/first.h

@@ -13,6 +13,12 @@
 #define _DEFAULT_SOURCE
 #endif
 
+#if defined(__APPLE__) && defined(__MACH__)
+#ifndef _DARWIN_C_SOURCE
+#define _DARWIN_C_SOURCE
+#endif
+#endif
+
 #ifndef __STDC_WANT_LIB_EXT1__
 #define __STDC_WANT_LIB_EXT1__ 1
 #endif