Fix RPATH for APPLE

Fix RPATH for APPLE

By setting the CMake minimum version to 3.1 CMake automatically
adds the correct magic to make the library relocatable on
the filesystem (instead of burning the location of the library
at link time into the binary).

ex:
otool -L bin/http-connect
bin/http-connect:
	@rpath/libevent_extra.2.2.0.dylib (compatibility version 2.2.0, current version 0.0.0)
	/usr/lib/libz.1.dylib (compatibility version 1.0.0, current version 1.2.8)
	/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1238.0.0)

Fixes: #468 (cherry-picked)
(cherry picked from commit cc0e04d798643b76ce1874213cb6cc3b2c6b5ac4)