Introduce $(build_shlibdir)

Abstracts away the fact that Windows wants shared libraries to be in $(bindir) whereas other operating systems prefer their shared libraries and executables separated
General cleanup of .dll installation

Also removal of .SILENT rule
Fix codespeed submission process
Don't ignore dll installation failure
Merge pull request #5727 from JuliaLang/sf/build_shlibdir

Improve dll installation logic
Whoops, wrong HTTP code.  Also, fix old error message