build: Endian compatibility

- Detect endian instead of stopping configure on big-endian
- Add `byteswap.h` and `endian.h` header for compatibility with
  Windows and other operating systems that don't come with them
- Update `crypto/common.h` functions to use compat
  endian header

configure.ac

@@ -350,8 +350,8 @@ if test x$use_lcov = xyes; then
     [AC_MSG_ERROR("lcov testing requested but --coverage flag does not work")])
 fi
 
-dnl Require little endian
-AC_C_BIGENDIAN([AC_MSG_ERROR("Big Endian not supported")])
+dnl Check for endianness
+AC_C_BIGENDIAN
 
 dnl Check for pthread compile/link requirements
 AX_PTHREAD
@@ -438,17 +438,22 @@ if test x$TARGET_OS = xdarwin; then
   AX_CHECK_LINK_FLAG([[-Wl,-dead_strip]], [LDFLAGS="$LDFLAGS -Wl,-dead_strip"])
 fi
 
-AC_CHECK_HEADERS([endian.h stdio.h stdlib.h unistd.h strings.h sys/types.h sys/stat.h sys/select.h sys/prctl.h])
+AC_CHECK_HEADERS([endian.h byteswap.h stdio.h stdlib.h unistd.h strings.h sys/types.h sys/stat.h sys/select.h sys/prctl.h])
 AC_SEARCH_LIBS([getaddrinfo_a], [anl], [AC_DEFINE(HAVE_GETADDRINFO_A, 1, [Define this symbol if you have getaddrinfo_a])])
 AC_SEARCH_LIBS([inet_pton], [nsl resolv], [AC_DEFINE(HAVE_INET_PTON, 1, [Define this symbol if you have inet_pton])])
 
 AC_CHECK_DECLS([strnlen])
 
-AC_CHECK_DECLS([le32toh, le64toh, htole32, htole64, be32toh, be64toh, htobe32, htobe64],,,
+AC_CHECK_DECLS([le16toh, le32toh, le64toh, htole16, htole32, htole64, be16toh, be32toh, be64toh, htobe16, htobe32, htobe64],,,
 		[#if HAVE_ENDIAN_H
                  #include <endian.h>
                  #endif])
 
+AC_CHECK_DECLS([bswap_16, bswap_32, bswap_64],,,
+		[#if HAVE_BYTESWAP_H
+                 #include <byteswap.h>
+                 #endif])
+
 dnl Check for MSG_NOSIGNAL
 AC_MSG_CHECKING(for MSG_NOSIGNAL)
 AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <sys/socket.h>]],