Allow building against the system sox library
Allow building against the system sox library

Provide SYSTEM_SOX=y to build against the system libsox instead
of downloading, building and linking staticly.

The default of linking staticly remains the same.


git-svn-id: 31cbfd89c62d6b8b302bb3be5c74ebef9d32ffd0@1250 01035d8c-6547-0410-b346-abe4f91aad63

tmiw/freedv-osx,tmiw/freedv-osx,tmiw/freedv-osx