install: Make the LD_LIRARY_PATH warning more aesthetic
install: Run ldconfig when installing on Unix. Closes #15596.

If ldconfig fails it emits a warning. This is very possible when installing
to a non-system directory, so the warning tries to indicate that it may
not be a problem.
Only run ldconfig on Linux
auto merge of #15597 : brson/rust/ldconfig, r=pcwalton

If ldconfig fails it emits a warning. This is very possible when installing
to a non-system directory, so the warning tries to indicate that it may
not be a problem.