Specify the -rpath linker flag when prefix != /usr.

This allows Asterisk to start without having to specify the
LD_LIBRARY_PATH. This can be disabled by passing --disable-rpath to
configure.

(closes issue ASTERISK-20407)
Reported by: David M. Lee
Review: https://reviewboard.asterisk.org/r/2132/
........

Merged revisions 379475 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@379477 65c4cc65-6c06-0410-ace0-fbb531ad65f3