Fix install makefiles for Solaris 10u10 builds

The install makefiles rely on the Solaris "df" utility to use a header entry spelled "avail" which worked in Solaris 10u6, but with Solaris 10u10, it is now spelled "Available". This caused FREE_SPACE to be empty, causing a silent if test in a sanity check to get a syntax error.

Changing this logic to use 'tail -1' and avoid the need to look at the spelling of the header lines.