Fixed build issues on MacOS caused by unset rpath

[LouisGameDev]

Similar to Linux, the rpath is now set to @loader_path, the mac version of $ORIGIN on Linux.

Now also moves *.dylibs (macs' version of dll) from lib to install.

[jmtatsch]

worked well for me, thank you!

[damageboy]

Worked for me as well!

[cryptomilk]

From a cmake point of view this is wrong. You should not manipulate the CFLAGS. On Linux you need to use:

set_target_properties(piper PROPERTIES BUILD_RPATH_USE_ORIGIN TRUE)

and and MacOSX:

set_target_properties(piper PROPERTIES MACOSX_RPATH TRUE)

according to: https://cmake.org/cmake/help/latest/prop_tgt/MACOSX_RPATH.html