libmetrics: hardcode MTU size to 1500 bytes if SIOCGIFMTU undefined
libmetrics: hardcode MTU size to 1500 bytes if SIOCGIFMTU undefined

fix compiling libmetrics in Tru64 (AKA OSF/1) that doesn't have SIOCGIFMTU
defined as reported in BUG75, reported to work with version 5.1b by
FURUSAWA Kazumi


git-svn-id: 27e0aca8c7a52a9ae65dfba2e16879604119af8c@1572 93a4e39c-3214-0410-bb16-828d8e3bcd0f

fastly/ganglia,fastly/ganglia,fastly/ganglia,fastly/ganglia,fastly/ganglia