Add back-up implementation of be32toh()

Older systems may not have the be32toh function defined. Check for this
and fall back to checking the endianness and calling bswap_32 directly
if needed.  This works on both old and new systems.

[Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>:
address comments raised by Lucas De Marchi [1], update commit message]
[1] http://www.spinics.net/lists/linux-modules/msg01129.html
diff --git a/configure.ac b/configure.ac
index 7781ce1..cd676bc 100644
--- a/configure.ac
+++ b/configure.ac
@@ -53,6 +53,9 @@
 # musl 1.0 and bionic 4.4 don't have strndupa
 AC_CHECK_DECLS_ONCE([strndupa])
 
+# RHEL 5 and older do not have be32toh
+AC_CHECK_DECLS_ONCE([be32toh])
+
 # Check kernel headers
 AC_CHECK_HEADERS_ONCE([linux/module.h])