Let's say we want to substitute libGL or any other lib. This can be
achieved by setting the enviroment variable LD_LIBRARY_PATH. However,
DT_RPATH has a higher priority, and if libGL can be found within the
directories listed in DT_RPATH, LD_LIBRARY_PATH then get's ignored.

DT_RUNPATH is exactly the same as DT_RPATH, except that it has a lower
priority than LD_LIBRARY_PATH.

I've encountered this problem with several tests. DT_RPATH already
contained a path to "/usr/local/lib", where a version of mesa could be
found. Hence, when I tried to substitute it with my custom build, I got
library mismatch resulting in segfault.

On some systems DT_RUNPATH is already being used by default, but it
will be safer to explicitly specify this behavior.

Signed-off-by: Yevhenii Kolesnikov <yevhenii.kolesnikov@globallogic.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>