configure.ac: Build Windows resources iff building for Windows host
configure.ac: Build Windows resources iff building for Windows host

Don't assume we need them just because windres is available.  Do assume
that windres is available when building for Windows.

https://lists.andrew.cmu.edu/pipermail/openslide-users/2016-July/001274.html

openslide/openslide,openslide/openslide,openslide/openslide,openslide/openslide