Use the right header guard.

Believe it or not, clang found this detail with -Wheader-guard.
darwin13 also uses kernel_sysctl
Use vm_region_64 on 64-bit arch; cache memory for longer

This is a port of the Apple patches at
http://www.opensource.apple.com/source/net_snmp/net_snmp-140.1/patches/darwin64.patch
1. Use vm_region_64 instead of vm_region on __ppc64__ or __x86_64__
2. Use 300 second timeout for the cache.

This comment about the code being incredibly expensive is
very true: on a 1.7 GHz i7 with 8 GB of RAM (admittedly a
small machine) it took 17 seconds to calculate and
return HOST-RESOURCES-MIB::hrMemorySize.0.  Most SNMP
clients would give up long before getting this answer.
If sdl_alen is too long for the static buffer, call it incomplete instead of overrunning the buffer