externalpackages.cmake: Fix static boost to not overlink (#2537)
externalpackages.cmake: Fix static boost to not overlink (#2537)

When compiling OIIO against a static version of boost where only a subset of boost libs have been built:
(eg, boost configured with --link=static --with-filesystem --with-system --with-thread --with-headers)
We want to disable the automatic MSVC linking, which will otherwise also try to link against unused and non-existant boost libraries (such as date_time).
lgritz/oiio,OpenImageIO/oiio,OpenImageIO/oiio,lgritz/oiio,OpenImageIO/oiio,OpenImageIO/oiio,lgritz/oiio,lgritz/oiio