Fix having to set LD_LIBRARY_PATH on Linux

[schmoelder]

On Linux, I always have to set LD_LIBRARY_PATH in order to run CADET.

@mbeyss mentioned this is usually a sign of some mal-configured cmake file and it should be fixed.

[schmoelder]

This works on my machine. @sleweke this should also capture APPLE, right?

if (UNIX)
	# The RPATH to be used when installing, but only if it's not a system directory
	set(CMAKE_INSTALL_RPATH "${CMAKE_INSTALL_PREFIX}/lib")
	list(FIND CMAKE_PLATFORM_IMPLICIT_LINK_DIRECTORIES "${CMAKE_INSTALL_PREFIX}/lib" isSystemDir)
	if ("${isSystemDir}" STREQUAL "-1")
		set(CMAKE_INSTALL_RPATH "${CMAKE_INSTALL_PREFIX}/lib")
	endif()
endif()