Made GC_DISK_HEADROOM configurable.

Added --gc_disk_headroom to mesos-slave which let's users configure
the disk headroom value used to calculate maximum executor age.

Review: https://reviews.apache.org/r/28681
Modify mesos configure.ac to set $with_svn to `brew --prefix subversion`
for OS X. Fixes MESOS-2113.
Modify libprocess configure.ac to set $with_svn to `brew --prefix subversion`
for OS X. Fixes MESOS-2113.