Support operating systems without some non-POSIX headers

The arpa/nameser.h, netinet/ip.h, and resolv.h headers are not crucial
to building LibreSSL. The netinet/ip.h header is used in nc(1) for
optional IPTOS_ features that can be ifdef'd on systems without support.

The endian.h header is the upcoming standard header and should be used
whenever available and correct. The machine/endian.h header is
non-standard and doesn't have to exist on POSIX systems.

Fix the check for getpagesize(3) not forward declaring the function,
such that CFLAGS with -Werror=implicit-function-declaration doesn't
cause the check to fail.

include/compat/arpa/nameser.h

@@ -4,7 +4,9 @@
  */
 
 #ifndef _WIN32
+#ifdef HAVE_ARPA_NAMESER_H
 #include_next <arpa/nameser.h>
+#endif
 #else
 #include <win32netcompat.h>