The executive summary is that in Mac OS X 10.11 (El Capitan) System Integrity Protection (SIP) was enabled such that the value of the DY_LD_LIBRARY_PATH variable is ignored.
"Spawning children processes of processes restricted by System Integrity Protection, such as by launching a helper process in a bundle with NSTask or calling the exec(2) command, resets the Mach special ports of that child process. Any dynamic linker (dyld) environment variables, such as DYLD_LIBRARY_PATH, are purged when launching protected processes."
