Autoconf updates

Move machine/endian.h to endian.h, use AC_HEADER_RESOLV over individual
header checks, and include prerequisites for netinet/ip.h check.

include/compat/endian.h

@@ -1,6 +1,6 @@
 /*
  * Public domain
- * machine/endian.h compatibility shim
+ * endian.h compatibility shim
  */
 
 #ifndef LIBCRYPTOCOMPAT_BYTE_ORDER_H_
@@ -22,7 +22,7 @@
 #endif
 
 #elif defined(HAVE_ENDIAN_H)
-#include <endian.h>
+#include_next <endian.h>
 
 #elif defined(__sun) || defined(_AIX) || defined(__hpux)
 #include <sys/types.h>
@@ -32,9 +32,6 @@
 #include <standards.h>
 #include <sys/endian.h>
 
-#else
-#include_next <machine/endian.h>
-
 #endif
 
 #ifndef __STRICT_ALIGNMENT