LD_LIBRARY_PATH contains an empty entry during build

[glandium]

I was looking at the output of x.py with -vv for $reasons, and noticed that commands run with LD_LIBRARY_PATH set to multiple paths, separated with a colon, which is normal, but the last character is a colon, which effectively adds the current working directory in the list of paths used. This is generally considered bad practice.

Cc: @alexcrichton

[ExpHP]

Example of how this can cause things to go wrong:

pulldown-cmark/pulldown-cmark#122

[glandium]

FWIW, when LD_LIBRARY_PATH is set before running x.py, its original value ends up at the end of the LD_LIBRARY_PATH that had a trailing colon.

[glandium]

It looks like it's a cargo issue. Moving there.