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>
 

include/compat/machine/endian.h

@@ -21,7 +21,7 @@
 #define BYTE_ORDER BIG_ENDIAN
 #endif
 
-#elif defined(__linux__) || defined(__midipix__)
+#elif defined(HAVE_ENDIAN_H)
 #include <endian.h>
 
 #elif defined(__sun) || defined(_AIX) || defined(__hpux)

include/compat/netinet/ip.h

@@ -8,7 +8,9 @@
 #endif
 
 #ifndef _WIN32
+#ifdef HAVE_NETINET_IP_H
 #include_next <netinet/ip.h>
+#endif
 #else
 #include <win32netcompat.h>
 #endif

include/compat/resolv.h

@@ -12,7 +12,7 @@
 #else
 #include <../include/resolv.h>
 #endif
-#else
+#elif defined(HAVE_RESOLV_H)
 #include_next <resolv.h>
 #endif