check_disk is missing -lrt on Solaris

The following is being reported against monitoring-plugins-v2.1.1-39-g8095.

In the link of check_disk.o on Solaris 8, I get:

Undefined                       first referenced
 symbol                             in file
nanosleep                           check_disk.o
ld: fatal: Symbol referencing errors. No output written to check_disk
*** Error code 1
make: Fatal error: Command failed for target `check_disk'

A look at the man page for nanosleep(3RT) on this platform shows that
it requires the -lrt library to be linked in, to access this function.