Remove Windows special-casing for openlibm and openspecfun installation (and bump them)

openspecfun now needs headers from openlibm, and these were not getting installed on Windows

slightly messy since this leaves versioned dll's in usr/lib

ref #6063 and #6295

cc @ihnorton @nalimilan


---
On second thought it might be better to do this directly in openlibm and openspecfun, add a `shlibdir` variable to `Make.inc` in both cases, and skip the version-numbering steps on Windows. Opinions?