linux/rpath: fix relative path finding for symlinked dirs

[pmolodo]

This fixes linking issues if you happen to have a symlink as part of your build path.

Previously, mayaUsd_add_rpath would use "realpaths" when comparing to origin, but mayaUsd_init_rpath would ONLY use a realpath if the argument was originally a relative path. Thus, if you fed in an absolute path with a symlink, you would end up comparing a symlink path to a realpath.

[pmolodo]

This addresses the note mentioned here:

#198 (review)
#198 (comment)

[kxl-adsk]

@elrond79 what's the simplest way to reproduce the issue using our build script?

[pmolodo]

This was on Linux - add args (ie, --pxrusd-location, etc) as needed to find your dependencies.

# The problem seems to crop up on rebuilds - to simulate, we pre-create this dir
mkdir -p /tmp/dir1/dir2/dir3/workspace/install/RelWithDebInfo/plugin/al/lib
ln -s /tmp/dir1/dir2/dir3 /tmp/symlinkDir
python $MAYA_USD_SRC/build.py /tmp/symlinkDir/workspace -v 3 --stages configure,build,install

The above build should appear to complete successfully - however, if you inspect some of the libs, you'll see they have linking problems - ie:

...which should show:

libAL_USDTransaction.so => not found

You can also inspect the RUNPATH directly, to see that it's going up the wrong number of directories:

readelf -a /tmp/symlinkDir/workspace/install/RelWithDebInfo/plugin/al/lib/python/AL/usd/transaction/_AL_USDTransaction.so | grep RUNPATH