Put -L paths from scripts are in LD_LIBRARY_PATH

Like the old build command, this commit adds support for executables run by
cargo (and tests) to ensure that `-L` paths for a dynamic library are added to
LD_LIBRARY_PATH after a compilation is run.

Closes #915
auto merge of #919 : alexcrichton/cargo/issue-915, r=brson

Like the old build command, this commit adds support for executables run by
cargo (and tests) to ensure that `-L` paths for a dynamic library are added to
LD_LIBRARY_PATH after a compilation is run.

Closes #915