include <sys/endian.h> according by checking in meson

The <sys/endian.h> header has existed in macOS since around ~26. This
causes the `htobeNN`/`htoleNN` macros to be redefined in <isc/endian.h>
in terms of <libkern/OSByteOrder.h> when other system headers include
<sys/endian.h>.

Fix this issue by using checking for the existence of <sys/endian.h> in
meson and including it according to the probe result.

lib/isc/include/isc/endian.h

@@ -13,8 +13,7 @@
 
 #pragma once
 
-#if defined(__DragonFly__) || defined(__FreeBSD__) || defined(__NetBSD__) || \
-	defined(__OpenBSD__) || defined(__bsdi__)
+#ifdef HAVE_SYS_ENDIAN_H
 
 #include <sys/endian.h>
 
@@ -32,7 +31,7 @@
 #define le64toh(x) letoh64(x)
 #endif /* !be16toh */
 
-#elif defined __APPLE__
+#elif defined(__APPLE__)
 
 /*
  * macOS has its own byte-swapping routines, so use these.

meson.build

@@ -538,6 +538,7 @@ foreach h : [
     'regex.h',
     'stdckdint.h',
     'sys/mman.h',
+    'sys/endian.h',
     'sys/select.h',
     'sys/sockio.h',
     'sys/sysctl.h',