Use autoconf to check for sys/endian.h.

configure.ac

 dnl library checks.
 dnl ================================================================
 
-AC_CHECK_HEADERS([endian.h])   dnl Linux
-AC_CHECK_HEADERS([fcntl.h])    dnl POSIX
-AC_CHECK_HEADERS([getopt.h])   dnl POSIX
-AC_CHECK_HEADERS([locale.h])   dnl C89
-AC_CHECK_HEADERS([stddef.h])   dnl C89
-AC_CHECK_HEADERS([stdbool.h])  dnl C99
-AC_CHECK_HEADERS([sys/time.h]) dnl POSIX
-AC_CHECK_HEADERS([wchar.h])    dnl C89
-AC_CHECK_HEADERS([wctype.h])   dnl C89
+AC_CHECK_HEADERS([endian.h])     dnl Linux
+AC_CHECK_HEADERS([fcntl.h])      dnl POSIX
+AC_CHECK_HEADERS([getopt.h])     dnl POSIX
+AC_CHECK_HEADERS([locale.h])     dnl C89
+AC_CHECK_HEADERS([stddef.h])     dnl C89
+AC_CHECK_HEADERS([stdbool.h])    dnl C99
+AC_CHECK_HEADERS([sys/endian.h]) dnl BSD
+AC_CHECK_HEADERS([sys/time.h])   dnl POSIX
+AC_CHECK_HEADERS([wchar.h])      dnl C89
+AC_CHECK_HEADERS([wctype.h])     dnl C89
 
 AC_TYPE_SIZE_T
 AC_TYPE_SSIZE_T

src/include/compat/endian.h

 #if defined(HAVE_ENDIAN_H)
 #	pragma GCC system_header // Silence "warning: #include_next is a GCC extension"
 #	include_next <endian.h>
+#elif defined(HAVE_SYS_ENDIAN_H)
+#	include <sys/endian.h>
+#	if !defined(be16toh)
+#		define be16toh(x) betoh16(x)
+#	endif
+#	if !defined(le16toh)
+#		define le16toh(x) letoh16(x)
+#	endif
+#	if !defined(be32toh)
+#		define be32toh(x) betoh32(x)
+#	endif
+#	if !defined(le32toh)
+#		define le32toh(x) letoh32(x)
+#	endif
+#	if !defined(be64toh)
+#		define be64toh(x) betoh64(x)
+#	endif
+#	if !defined(le64toh)
+#		define le64toh(x) letoh64(x)
+#	endif
 #elif defined(__APPLE__)
 #	include <libkern/OSByteOrder.h>
 
 #	define __BIG_ENDIAN    BIG_ENDIAN
 #	define __LITTLE_ENDIAN LITTLE_ENDIAN
 #	define __PDP_ENDIAN    PDP_ENDIAN
-#elif defined(__OpenBSD__)
-#	include <sys/endian.h>
-#elif defined(__NetBSD__) || defined(__FreeBSD__) || defined(__DragonFly__)
-#	include <sys/endian.h>
-
-#	define be16toh(x) betoh16(x)
-#	define le16toh(x) letoh16(x)
-
-#	define be32toh(x) betoh32(x)
-#	define le32toh(x) letoh32(x)
-
-#	define be64toh(x) betoh64(x)
-#	define le64toh(x) letoh64(x)
 #elif defined(_WIN16) || defined(_WIN32) || defined(_WIN64)
 #	if BYTE_ORDER == LITTLE_ENDIAN
 #		include <winsock2.h>