bf_intern_openvdb makes a significant number of template instantiations
causing it go over the maximum number of sections (int16) in a coff file
when doing a debug build.
This change switches the compiler to use the extended coff format which
has this field extended (int32) all linkers post msvc2005 can process
this format so there's no reason not to turn this on globally.
Clang on windows does not need this change since clang switches implicitly
to the extended format when required. [1]
[1] https://reviews.llvm.org/rL217812
f4d548d384